An idea!

A good way to combine the strategy & battle game modes is to have a limited number of power supplies. Thats these things:

Players then use the power supplies to build the battle machines that they need... but the power supplies are also needed to power the factories that build and maintain the machines. So players can design the factories with defenses so they are like fortresses. The factories and battle machines are considered pretty much equal then in the strategy game mode, and the strategy and battle modes are tied together much more nicely.