NOTE: THIS WORKS ON THE ORIGINAL RELEASE OF THE GAME AND, UNFORTUNATELY HAS BEEN PATCHED WITH THE MORE RECENT TITLE UPDATE.
By using a deceptively simple trick you can make the Strength, Critical Chance, and Critical Damage stat bonuses from the Unstoppable wrath power permanent. Using the same method the Arcane and Arcane Critical Chance bonuses from Aegis Guard can also be made permanent. Unfortunately, the Defense and Resistance bonuses aren't made permanent this way.
Simply activate the desired effect and save and reload before the effect wears off. The game will essentially forget to tell the stat bonuses to turn off. Buy a respec from Vulgrim and spend your points somewhere else and never worry about managing your combat buffs again! If you ever want to undo the effect, just cast the Unstoppable power again and the stats will revert as soon as it wears off.
I don't know if anyone else has figured this out (I'm sure they have), but it felt pretty cool discovering it for myself last night and testing out the various use cases. By the way, I'm aware of the irony of properly testing a bug.
UPDATE: Thanks to user CaptnMexico21, it is possible to stack the effects a limited number of times with Unstoppable and Aegis Guard. Each skill has three levels that get upgraded when you put a skill point into it and each of these levels individually counts as a separate instance of the exploit. So, if you cast the level 1 version of Unstoppable, save and reload, then put the skill points necessary to increase it to level 2 and then save and reload the effects will stack.
This means that the stat boosts can be stacked three times, one for each level the skill are at in the skill tree. This does make the game rather hilariously easy, even on Apocalyptic, so use it at your own discretion.