Use the freaking cricketot you get earlier in the game.
It gets bide sometime around lvl 13 or 15 and use it to wreck havoc on the tangrowth.
i mean i ONE HIT IT LIKE WTF
and the third pulse tangrowth is the easiest if you dont count the chatot at the end spamming chatter... i still OHKOed it with a bite... from a lvl 30 pokemon
wtf this game is so unbalanced