You can get gold with marauder and gift or phantom image spell.
Upgrade or supress an item and leave 1-2 friendly gremlins as a troop source, Kill those troops and loot. Recharge mana with green dragons, chargers.
In 30 minutes you can get ~1 000 000 gold.
In demonis you will get more gold (2000-18000 gold/use of marauder skill) than in other areas (demonis have a 95 dificulty)
Other option (without upgrade items) is with summon thorns and red dragons fire ability, or trap spell to kill thorns in a normal combat. I think it works with demon portal too, or phoenix.
And do not use autocombat
