Hello there,newbie here. 
So, I read most posts on this thread and I wanted to share my experience with the so called Tangrowth boss. 
At the first encounter I took him down to very low hp without even using any particular tact.However when I reached that point all my pokemon were dead. 
So, at the second try all I used was a Meowth(lvl 18) and a Swoobat (lvl 21). 
I started with Meowth by using Fake Out and immideately Screech. 
He then 1 hit ko it with one ability I don't recall.(Mega Drain or the poison attack i assume) 
Right after i switched it with Swoobat. 
1st attack  : Heart Stamp - Tangrowth flinches. 
2nd attack : Heart Stamp again. 
He then used an abilitity that boosts his Special Attack/Defense. 
3rd attack : Confusion and he is dead. 
Lastly, the abilities used(Fake out,Heart Stamp x2 and Confusion) DID NOT crit. 
P.S. Swoobat's abilities Confusion and Heart Stamp are super effective on Tangrowth.