Elements Unite! You will start the duel with 500 Life Points and your starting hand will be 1 card, which is "Gate Guardian". In addition, you will start the Duel with Suijin, Kazejin, and Sanga of the Thunder on the side of your field. Their effects are negated and they cannot attack as long as they are face-up on the field.
The Gate Guardian given to you by your skill will be all you need to stall so summon it immediately.
Tristan can deck out quickly because of needle bug nest, so we use a lot of draw effects in this deck. Shard of Greed, Legacy of Yata-Garasu, Jar of Greed, Good Goblin Housekeeping, Thunder Dragon, etc. helps you thin your decks quicker.
You can discard Thunder Dragon to search out 2 more copies from your deck then return one of the Thunder Dragon you searched to the deck with Good Goblin Housekeeping. You can then discard the Thunder Dragon in your hand to search the Thunder Dragon you returned to the deck.
During your final turn summon Gravekeeper's Vassal and activate Secret Pass to the Treasures and 3 Union Attack on him. Attack Tristan Directly to end the Duel.
Notes [Previous Version]
Special summon Gate Guardian immediately. He will be enough to stall the entire duel.
You can attack Tristan's monsters, but do not attack face-up attack position monsters while Tristan has a set spell/trap card. The set card could be Battleguard Howling and cause you to lose the duel.
Activate Sogen to boost Gate Guardian's attack.
Equip Wonder Wand to Gravekeeper's Recruiter and activate it's effect to thin your deck by 3 cards. It is best to search another Gravekeeper's Recruiter with his effect if possible.
Shard of Greed, Good Goblin Housekeeping, and Jar of Greed is also used to help you thin your deck.
During your final turn, Normal summon Gravekeeper's Vassal and then special summon Caligo Claw Crow. Use Secret Pass to the Treasures and 2 Union Attack on Gravekeeper's Vassal.
Use Unhappy Girl to stall the duel by using her to attack into Tristan's monsters. You will not recieve any damage because of Holy Guard and Tristan's monster would not be able to attack or change battle position because of Unhappy Girl.
Since Holy Guard only protects your LP during your turn, you should activate Spirit Barrier to protect yourself in case Tristan attacks Unhappy Girl.
Set Crystal Seer to excavate your deck when she is flipped face-up. If all of Tristan's monsters cannot attack, you can flip Crystal Seer face-up yourself to activate her effect then crash her into a monster to get rid of her.
During your final turn if you have 2 Unhappy Girls, you can tribute one of them into UFOroid to make room for your fusion monster.
Fusion summon UFOroid Fighter using Gate Guardian and UFOroid and then use Double Summon to bring out Gravekeeper's Vassal.
Sorry last anon with Elements: But that's totally BS.
1) It isn't Joey(and that is the first anon asking for)
2) The mission states no damage, but you start with damaged HPs = mission fail from the start.
I used a Fusion deck, but be aware that the lvl 30 has block attack and Garma Sword can destroy everything <2.6k ATK/DEF.
I beat tristan with aromage garden + ra deck
Still can get 8k score