Senna in her current state is absolutely broken. Her self heal, which scales with both AP and AD (which scales infidelity!), combined with the fact excess crit counts as lifesteal makes her an absolute monster mid-late game. She can easily 1v5 against burst heavy teams. God forbid she gets a red buff, she can go from 10-100% hp in a Q or two. This, combined with the fact her lifesteal, range, and ad scale infinitely, along with her global ult, makes her an absolute powerhouse of a champion.
She should not be able to self sustain. Get rid of her self heal, or at least make her self heal only when she targets an ally. And the self heal, if it dosent get outright removed, needs to be reduced.