Thanks, I'll try it. I haven't played for about 5 days now, since I got stuck there

Added after 5 hours 42 minutes
phew, I made it..
At first, I took out the weakest ones with humans, and those slightly stronger with a Necromancer.
Then the invader heroes sailed in; I killed them, then built everything up in the city, gathered an army, cleared the whole island - then sailed off to clear all the small islands, killing the last 2 heroes on ships along the way. After that, I landed on the second large island with a huge army - and wiped everyone out .. ) Captured Mythcity, built up the citadel, arrived at the mages via the portal - and killed them :)