Basically PvE stars out button mashing. You can kill/attack/use skills/move in any which way or order and you can progress through the game. After this comes getting new skills and capping your first elite skill. At this time some synergy is formed with your build (synergy with henchmen and heroes comes later). Also here is the point where runes are added. Soon after this point you begin to sync heroes builds and then heroes builds with each other and your own. After that point you load your own armor up with as many runes as possible and cap more leet skills. Focus-firing and calling targets follows soon after. The last step is basically super powerful bosses and AI that make you think on your feet, but high end PvE never gets that hard

. If you wanted to upgrade your heroes armor I would say that it is optional, I completed 4 different campaigns with 4 different characters of which none were upgraded, so ya it's possible.
Oh and by the way, right before high end PvE you will need to squire different weapon sets for different circumstances. Heroes probably don't need these though.