I have this very tradition that whenever I'd get a "new" laptop, I'd finish the first 4 epic war games to 100% completion (minus the ew3 cave of trials 4 and 5)
easier strategy than arda's: fend off with 2-3 paladins/beast riders/protectors, hold the line with the arrow tower, build up the mana pool then send 1 or 2 angels (NOT at the same time and wait a bit in between) to shave the fortress's life off bit by bit so that the boss isn't joined by 100+ enemies
make sure you have barracks, archery and academy upgraded to the max, as well as arrow tower to lvl 4. Send 2 paladins whenever you're feeling overwhelmed, then upgrade the mana pool when you get the opportunity. Make sure you kill the totems first. Wash, rinse, repeat.
NOTE: this strategy works for all races