why is garen allowed to do 1000 true damage ult when half hp???

full AD garen does assassin damage with a tanky output here i thought if im nasus i can go armor and hp to outlast his damage but nope at half hp lvl 16 its a death sentence to even exist even though im going full tanky. he shouldnt be allowed to have a smite vs champions. i mean where is the counterplay at all here. so am i not allowed to exist at all near him regardless of class roles??? btw 450 base damage true damage on ult plus 30% missing hp like thats even fair at all... after dealing assassin burst with no kind of armor or hp items to save me.