I cannot finish the last level. After I kill Avatar I am attacked by about 30-50 heroes and they destroy me. i cannot convert Avatar since the imps cannot cross the bridge. In fact nobody attacks him - I can only kill him with Lightning. The same is in the North. Minions don't give a shit about call to arms and Imps wont cross the water and convert tile.

And even if I could get the Avatar body to convert him it does not matter since when he dies I am attacked instantly.

You can convert the avatar if you get him into your own dungeon before you take him down. For example with the Witches 'wind' spell or the Dragons 'Word of Power' spell. Killing him with lightning is expensive, dragons and reapers can get to him across lava.

He's very strong, but you don't need him though. You just have to win the final fight. Some hints:
- Build (boulder) traps around the hero-gates.
- Use the scavenging room to get a huge army of dragons and bile demons
- Don't fight all heroes at once, but just a few at a time with overwhelming numbers
- Wall in the group of heroes that spawn in the south, to the right of your gems.
- Get both horned reapers and all of the tentacles.