This achievement I will say is complete Luck! There is no way to tell when the Treasure Yeti will show or not, so I am giving some pointers out on what you can try to find him and collect his 7 Treasures.
An easy way to get this achievement is to play a solo Garden Ops game on Sharkbite Shores on Easy. Play as a normal Peashooter and defend 2 waves without planting any potted plants. If you don't get the Treasure Yeti on Wave 3, quit and try again, repeat over and over until you get the Treasure Yeti on Wave 3. Once he does show up, DON'T kill him, just follow him and keep shooting him and his bag until he's almost dead. Your goal is to find 1 or 2 Diamonds that'll drop before his death, but let him escape. If he gets near the water, be very careful as his diamond or himself can fall into the water and disappear on you. As long as there's still a Soldier Zombie or 2 around, the wave won't end and you can jump in the water to get knocked out. If you want to make sure the wave doesn't end, jump in the water before the timer runs out for his escape. Do a team retry and you get more chances! If you're lucky, you can get all 7 Diamonds off of 1 Treasure Yeti. Some say it glitches to where the wave will end anyways, so to try to prevent that from happening,
1: Don't Kill Yeti
2: Don't fool around with the water when attacking him
3: Make sure to drown yourself BEFORE the timer runs out
4: Make sure you are a Regular Peashooter to avoid poisoning or other ways to kill without trying
Hope this helps, good luck to you all!