Simple: if more HP is not enough, then EVEN MORE HP 8)

Or there's a formula used by Baldur's Gate: all damage done by enemies is doubled.

Maybe a combination of both...?

There's a mod that does this: randomly promoting enemies to "elite" rank and giving them random permanent buffs. Dragon Age Inquisition's Trials has a similar feature that can be toggled on/off.

Or add a global +1 or +2 to all enemies' level. This can be seriously broken though, I'm not sure.

"We make our choices and take what comes and the rest is void."