Levels 5 - 6 = 1 sacrifice
Levels 7 - 9 = 2 sacrifices
Fusion Monsters (purple monster cards) = right cards are needed along with Polymerization. Got it now?

Who's here? - Tristan, Te`A, Yugi, Joey, and Bakura.

Tristan - is probably the easiest duelist in the game. His cards aren't that strong at all, and he's great to practice with if you need fine-tuning. Just have at least one monster that needs one sacrifice and a few with ATK under 1500 and you'll do fine.
Cards he has: None really worth mentioning except for Soul of the Pure, which recovers 800 LP.
Cards to use: Any with ATK between 1500 - 1800 should do. Traps are nice but not needed.

Te`A - Once again, she should be cake as long as she doesn't have a chance to summon monsters who need sacrifices. If she does manage to get a strong monster on the field, try to defend or outATK her.
Cards she has: Magician of Faith is a pain is she used Monster Reborn and wants to get it again. Other than that, nothing worth mentioning.
Cards to use: Same as Tristan, but if defense is needed, try Prevent Rat or Mystical Elf. Again, traps aren't needed.

Yugi - is a little harder since he has great monsters like Summoned Skull and Dark Magician. However, he needs to sacrifice to use them. If he manages to get strong monsters out, use traps and defenses to counter him. If that doesn't work, you're at a loss.
Cards he has: Kuriboh is a pain because of his ability to block Direct damage. However, it can only be activated once, so it's not too bad. Swords of revealing light is not a good sign, as Yugi then has time to get his Dark Magician out. Beware his stronger monsters.
Cards to use: Change of Heart should come in handy. Swords of Revealing Light, Dark Hole, Trap Holes, and Summoned Skull will be great! Try to use level 4 monsters all the way unless you need to sacrifice.

Joey - Okay, he's tough compared to the first three. He has a lot of traps and strong monsters. Expect him to hit you right off the bat with a monster with 1500+ ATK points. I had real trouble with him, no matter how strong my monsters were.
Cards he has: Watch out for Kunai with Chain, Time Wizard - Baby Dragon combo, Trap hole, and Red Eyes Black Dragon.
Cards to use: Change of Heart, Swords of Revealing Light, Trap hole, and defense cards like Prevent Rat. Other than that, hit him with good basic monsters.

Bakura - Watch out for this little firecracker! He loves zombie and magician cards, and sicks a powerful card on you any chance he has. Once again, defense is good early in the duel, but be sure to play some stronger Lv. 5 - 6 monsters in this duel.
Cards he has: Deadly little deck. He has Change of Heart, Man Eater bug (auto destroys a monster when flipped up), two Dark Elves, and a Gemini Elf. Also, beware of his Spellbinding circle trap.
Cards to use: Change of Heart, Swords of Revealing Light, and any traps (like Trap hole or Mirror Force) and Lv. 5 - 6 monsters you have.

To get to second tier: Defeat each Tier 1 duelist twice.
Booster packs: 1 new one for reaching Tier 2, can obtain one after you beat Yugi and/or Joey 20 times.

Second Tier Walkthrough!

Wow! Mirror Forces and Jinzo seem to be introduced here. Nothing important.

Who's here? - Espa Roba, Mako, Rex, Weevil, Mai.

Espa Roba - He's a little harder than Bakura. He has a mixed bag. Meaning, I can't tell you much about his deck. Don't let his psychic ooga-booga act scare you. He's not as tough as you'd think.
Cards he has: Jinzo is quite the nuisance, as it renders your traps useless. He also has Mirror Force.
Cards to use: Change of Heart, Swords of Revealing Light, Magic Jammer, Mystical Space Typhoon, and Dark Hole. Try to take out his monsters before he can play Jinzo.

Mako Tsunami - Here's a worthy opponent! He uses mostly AQUA monsters, and sometimes forces cards back into your hand. Best bet? Use good Lv. 4 monsters, so that if they get sent back to your hand, you won't ned to sacrifice next turn to play them again.
Cards he has: Karyushin, Penguin Soldier (forceback card), Umi, Mirror Force, Trap Hole.
Cards to use: Change of Heart, Trap Hole, Dark Hole, Mystical Space Typhoon, Monster Reborn. Also, monsters like Axe Raider, Neo the Magic Swordsman, and Dunames Dark Witch also help.

Rex Raptor - Rex is easy only because he relies mostly on brute force to win. He has almost no traps except for Mirror Force. Use this to your advantage. Defend only with Prevent Rat or Mystical Elf, and attack like crazy. Lv. 5 monsters like Summoned Skull should blast him to pieces!
Cards he has: Mirror Force, one Trap Hole, one Mystical Space Typhoon, and just dinosaur monsters.
Cards to use: Swords Of Revealing Light, Trap Hole, Mirror Force, Monster Reborn, and Magician of Faith monster. Man Eater Bug helps too, but DO NOT use Milus Radiant, as it powers up Rex's monsters too.

Weevil - Ah, but what a pain this little nut is. He's strong, ruthless, and sneaky. He has traps out the wazoo, and his monsters are strong and often times have very bad effects for you. Beware his Insect Queen, and Parasite Parasite card, which makes all your monsters bugs.
Cards he has: Insect Queen (gains 100 ATK for every insect on either side of field), Parasite Parasite (makes all monsters bugs), Forest, Man Eater Bug, Needle Worm (if flipped up, you discard five cards from your deck).
Cards to use: Trap Hole x 2, Dark Hole, Mystical Space Typhoon x 2, Magic Jammer, Despell, Change of Heart, Swords of Revealing Light, Monster Reborn, Mirror Force. Try to get Lv. 6 - 7 monsters out here. Weevil is one of the toughest in the game!

Mai - Hmm.. Mirror Force is big in Tier 2, and Mai is no exception for it. She is easier than Weevil, but can still be tough if she springs Harpie's Brother on you. Other than that, compared to Weevil, Mai's cake.
Cards she has: Harpie's Brother, Mirror Force, Magic Jammer, Harpie's Feather Duster, Monster Reborn.
Cards to use: Change of Heart, Mystical Space Typhoon, Magic Jammer, Pot of Greed, Magician of Faith, Man Eater Bug.

To get to third Tier: Beat each Second Tier duelist three times.
Booster packs: 1 for getting to Tier three, can obtain one for beating Mako and/or Mai 20 times.

Third Tier Walkthrough!

All that's here are Rare Hunters. But, oh what fun!

Who's here? - Lumis/Umbra, Arkana, Rare Hunter, Strings, Marik Ishtar.

Lumis/Umbra - These two dorks act tough, but are nothing compared to Weevil. Just beware their usual stall tactics.
Cards they have: Swords of Revealing Light, and various Mask cards.
Cards to use: Despell, Mystical Space Typhoon x 2, Heavy Storm, Change of Heart, Magic Jammer. Defense isn't needed here. Just attack as best you can.

Arkana - He's tricky, as he has a deck centered around the Dark Magician duo. Lots of traps, but you should be able to counter that with traps of your own.
Cards he has: Mirror Force, Monster Reborn, Dark Magician x 2, Dark Magician Girl, Trap hole.
Cards to use: Easy to counter with Magic Jammer, Change of Heart, Dark Hole, Trap Hole, Seven Tools of the Bandit, and Reaper of Cards monster. Swords of Revealing Light can be of help too.

Rare Hunter - this guy is pathetic with a capital P. He has mostly Exodia cards, but almost never can put all five correct pieces together. Easy as cake.
Cards he has: Swords of Revealing Light, Exodia set x 3.
Cards to use: Usual. Change of Heart and some Trap holes. Any monster with 1500 - 2500 ATK can easily beat this dude.

Strings - This guy is annoying! He's not a strong duelist, but he has cards that can restore his LP. He also has Mirror Force, but isn't too bad.
Cards he has: Numinous Healer x 2, Swords of Revealing Light, Mirror Force, Sebek's Blessing.
Cards to use: Mystical Space Typhoon x 2, Heavy Storm, Monster Reborn, Man Eater Bug. Try to get Lv. 5 - 6 monsters out as soon as possible.

Marik Ishtar - Well, now you're up against the first of 7 M. Item holders. Everyone else up to this point were easy. Marik is like a test to see how you fair against the other M. Item holders. He's very hard.
Cards he has: Deliquent Duo x 2, Man Eater Bug, Magician of Faith x 2, Gemini Elf, Vorce Raider x 2, Monster Reborn, Sebek's Blessing, Harpie's Feather Duster.
Cards to use: The best you've got in Traps, Magic, and Monster cards.

To get to Fourth Tier: Beat all Third Tier duelists 4 times each.
Booster packs: 1 for getting to Tier Four, 1 after beating Marik 20 times.

Fourth Tier Walkthrough!

Uh oh... Everyone here is very hard. Beware!

Who's here? - Yami Bakura, Ishizu Ishtar, Seto Kaiba, Shadi, Yami Yugi.

Yami Bakura - is a pain and should be dueled last. He's ruthless, but somehow seems pretty nice! Just try your best to get your best cards out when you need them, otherwise defend until you get a good stratedgy going.
Cards he has: Man Eater Bug x 2, Magician of Faith x 2, Gemini Elf, Vorce Raider, Mirror Force, Kuriboh, Magic Jammer, Change of Heart, and other traps.
Cards to use: Man Eater Bug, Magician of Faith, Harpie's Feather Duster, Mystical Space Typhoon x 2, Monster Reborn, Dian Keto the Cure Master (cures 1000 LP). Go slowly. Try to wear out his deck.

Ishizu Ishtar - Yup, she's Marik's sister. She's also strong, but you should be able to counter her.
Cards she has: Mirror Force, Maha Vailo, equipment cards.
Cards to use: Swords of Revealing Light, Change of Heart, and the usual traps you'd use. Lv. 4 - 5 monsters should be enough to knock her out.

Shadi - is incredibly easy! What's he doing with elite duelists? He'll try to wear out your deck, however, so be careful.
Cards he has: Needle Worm x 2, Morphing Jar series, Cyber Jar, Swords of Revealing Light.
Cards to use: Use stradegy like with Ishizu and you'll win.

Seto Kaiba - Easy and hard at the same time. Usually, he uses Cyber Stein to bring out his Blue Eyes Ultimate Dragon without sacrifices, but he'll need to give up 5000 LP to do so once!
Cards he has: BEUD, Cyber Stein, Mirror Force, Trap Hole, Change of Heart.
Cards to use: Use the stradegy for Yami Bakura and Kaiba should fall easily. But include a Dark Hole in case he manages to get out his BEUD.

Yami Yugi - Easy, easy, easy. He's a wimp compared to Yami Bakura. Just hit him hard and fast.
Cards he has: Magician of Faith, D. Magician x 2, D. Magician Girl, Change of Heart, Graceful Charity x 2, Mirror Force, Kuriboh, Trap Hole.
Cards to use: Just do your best. Use the stradegy listed above.

To get to fifth tier: Beat all fourth tier duelists 5 times each.
Booster packs: Obtain the Blue M. Puzzle pack for reaching fifth tier. Can obtain one by beating Yami Yugi 20 times? (didn't get this far, just a rumor.)

Fifth Tier Walkthrough!

Most of the duelists here can only be dueled by "unlocking" them. I haven't unlocked them all, but I will tell you what I do know.

Duel Computer - Very hard! Don't duel unless you have tons of destructive traps and monsters or if you think you need such an intense challenge. You don't need to unlock this duelist. He comes automatically.
Cards he has: Pretty sure he has the same deck as SETO KAIBA.
Cards to use: The best you've got.

Maximillion Pegasus (Pegasus J. Crawford in japanese version) - Can only be unlocked if you own a TOON WORLD card. He's hard if he gets Toon World and any toon monsters into play. But, if you can get rid of Toon World, he's cake.
Cards he has: Toon World, and toon monsters like Toon Skull, Toon Dragon, Toon Mermaid, and Manga Ryu-ran.
Cards to use: Armed Ninja, Mystical Space Typhoon or Harpie's Feather Duster/Heavy Storm, Mirror Force, Trap Hole, Swords of Revealing Light (shouldn't need it but nice addition anyway), and any monster with an ATK of over 1500 (Dunames Dark Witch and Vorce Raider are good since they need no sacrifice and both have close to 2000 ATK points.).

Simon - I think this guy can only be unlocked if you win the National Championships in November. Haven't unlocked him yet.

There is another guy, but I forgot his name, or how to unlock him. Sorry. Once I do, I will definitely tell you!^^;