I play as a Panth abuser for ranks,and NO GOD PLEASE NO.
"too easily countered by armor and health" you have to not let people get those things,just like the bully panth is.
"stun too short" iirc is 1 second or so,and is still enough to destroy anyone in lane or not prepared to face you,especially if you're not alone. If they are prepared,you still have high as fuck damage aand can win the trade unless it's a really really really fed adc,or a good adc with a janna.
"teamfight isn't op" you either go diving,trying to assassinate a carry and taking the attention of the enemies while your team slaughters (and thinks you're crazy if done in low elo),or you stay alert to help whoever is the main target of the enemies,stunning the threats and chunking them down before they can land anything on your allies. Or you do the "tank".
"falls of mid-late game pretty hard" Yeah,this is true,he's a bruiser bully with high base damaage and decent scalings,so his main strength is shutting down everyone he can as early as possible (if you haven't helped your jungler and/or one other lane to get at least a slight advantage,then you failed. If in the meantime you lost lane too,you fucked up,unless is someone who can outsustain and take the damage while farming like vlad or maokai). That's why you don't go full damage (i don't know how you build it,i haven't looked) but go off-tank,like a lee sin would.
"his mana management" it's the only thing keeping him from destroying everyone just by vomiting spears as soon as he can,and with corrupting if you are not against a sustainer (vlad ecc...) you can still don't care much about mana. Also,the problem is only in lane,after level 9 you don't feel it much unless you stay for 3 minutes straight fighting without a disengage or such.
I may be only silver,but i know enough to not ask to buff someone there is a reason to hate.