Increase AP Cost and nerf the damage output on some offensive source abilites should do.
eg:

• Mortal Blow:
2AP > 4AP
so you'll need at least 5~6AP to get in place and setting up to deliver a nuke.
OR
2AP > 3AP and nerf it damage output...HARD!
It's capable of nuking even bosses so nerf hammer definitely needed here.

• Overpower: 2AP > 3AP
also really strong atm to shutdown bosses in one turn, then the ongoing turns the boss will just eat a chain of CC , so it's pretty much game over for bosses on round one.

• Same goes for the other. I see they start giving alot of "meta" skill an increase in AP for the latest update so doing the same for these Source powers would solve the problem as well.