try these options.....

1 SPIDERS LOTS OF SPIDERS
2 TANKS LOTS OF TANKS
3 Troops be prepaired to move them around

There is 1 unit that can take down a battle fortress, boris is the one, once he destroyes the battle fortress, 5 guardian(or less) are left and Boris will machine gun em down.

Note: Make sure u keep boris on the move cuz the battle fortress will run u over., yes it even runs over tanks such as an Apocalypse tank.

I find it efective to just build a combo of apacolyps (sry for spelling) tanks and some spiders and send the tanks first. Most players will start to try to kill the tanks 1 by 1 by running over them or mass firing, once they begin to do this mass ur spiders at em once inside just try to speed up the deaths of the fortresses

use lots of back eagles but first sent some regular tanks to distractem