I'm having a go at this now myself. After the training fights and the two very weak Devilfish units on Debir, the next weakest unit was listed as Lethal!
So it was necessary to do some exploring of the other islands - I don't think it'd be possible to complete this challenge otherwise. Found 5 Demonologists on Scarlet Wind which I am very happy about. I'm wondering whether they only spawn in Mage games, as I didn't see any of them in my Warrior game. Demonologists are just so awesome - not so much the drain life/heal but summoning even 1 Executioner right amidst the enemy is just beautiful. Angel Avenger works well with this for maximum carnage.
I should have mentioned that I picked the Green dragon and set to work on digging up those treasure chests every battle. With 10 rage only at this stage the fights have to be managed correctly. Currently I am using the 4 Royal Thorns that always seem to be available on Debir, Demonologists, Inquisitors, Swordsmen and Royal Snakes. I know that leadership is going to be a big problem so I bought anything that added leadership as well as two points of Glory. So now I have 1700 leadership at level 6 and Debir is clear. The other thing to keep in mind is that there is the quest on Rusty Anchor where you just pick up bottles - this should be done very early.