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.