I think there are many cases where Larian did give enemies too many turns, though I haven't counted yet.
As for ways to get more actions:
1) Action Surge - level 2 Fighter ability (those are full actions, not just attacks)
2) Haste
3) Potion of Speed
There are also ways to turn bonus actions into full actions/spell casts:
1) Quicken Spell metamagic (for spell cast action only)
2) Illithid powers
If you have trouble hitting it would be important to try to debuff AC or get Advantage to your attack.
