Only used power armour once myself, when it was an obvious part of the scenario. One of my settlers used a suit that I neglected to remove the power core from, after that happened I stuck all the cores far away from the circle of suits!
The key I found that worked for me as far as melee combat is concerned is strike and move, rinse and repeat. Either most of the creatures in the game are naturally sluggish or having an eight agility really helped. Also, having a sustained dot on your weapon of choice is a nice option to choose, I had one of those plus stagger, which makes for an awesome duo of effects.