Power Overwhelming plus Abusive Sergeant on a Tentacle kills a Flamewreathed. Doing it on a possessed villager is even better. If you only have PO, trading a couple of little minions into the big minion is also good.
Don't be afraid to trade. Don't be afraid to use your PO to get a favorable trade.
https://www.youtube.com/watch?v=A_wvxmUNSe8
Here's a good little primer on how to play zoo. You don't want slow cards like Siphon Soul--that doesn't help your win condition. You want sticky minions and you want reach. Substitutions like Harvest Golem, Dark Iron Dwarf, even Reckless Rocketeer should be considered for cards you don't have access to.
I personally really like Crazed Alchemist in zoo. It kills a Mana Tide or Flame Tongue Totem straight up and helps you make favorable trades.
You can even combo it with Abusive Sergeant or Dire Wolf Alpha to make a temporary buff into a permanent one.