Nobody seems to be able to help me with level 13, but I hope this helps with level 8. Its the same old advice. You need to get more of the strategic buildings early on than the enemy and hold on to them by tucking a unit on the opposite side of it from the enemy. The enemy AI is fooled into firing at your unit through the strategic building. Enemy units will build up, but this will give you time to concentrate elsewhere and as long as you hold onto more strat buildings than the enemy you will be able to build up more units to take him out.