When it comes to reduction of cards, I do believe that slow is the way to go, and give it time to be tested.
For Phase tower I think removing the shortened vulnerability durations, as well as the damage vulnerability and leaving only a life point buff for each upgrade would be the best way to handle this. The power cost of PT is 60, yet you usually need 2+ units to deal with it. This still allows the potency of the card, while allowing it not to completely steam roll certain factions.
Mortar Tower, just widen the spray pattern from 10 meters to 20-25 meters, that should keep it in check and not as potent