The Tangrowths frustrated me, but I was able to take them out after a couple tries with Trubbish, as Tapu Fufu mentioned.
I don't remember much about Muk, other than the battle took me two tries because of ZEL's other Pokemon. Meowstic obliterated it.
PULSE Abra legitimately made me curse, and forced me to add an entire new Pokemon to my team (Vanilluxe). It got my vote.
I won against Avalugg with Swagger hax from Tauros. If it weren't for that, it probably would have been much more difficult.
Swalot was frustrating at first, because it had been a while since I last played and I forgot I had Vanilluxe & Mirror Coat. The battle became way easier after that.
Magnezone didn't even land a hit on me. It flinched after one SD-boosted Waterfall, so my Samurott two-shotted it.