Yo what do I do vs an assassin J4 fr

Yell DEMACIA, after which he will realise you are on his team and therefore leave you alone

Blow him up before he has a chance to kill you.

Side step.

What are you playing? We're moving into a duskblade meta and armor is essential at countering it.

generally blow up if adc or assassin otherwise dodge the Q(the knockup spear with armor shred) to nerf his damage tons.

Either full tank and his rotation can’t kill you

Or

Full damage and blow him up first

and watch as your team blows him up bc hes squishy af and he probably blew all his escapes to dive