I went with Moldok, for now anyway. Thanks for the advice.
Bosses:
Spider - easiest of the bosses, much the same as in TL. I used Royal Thorns for this one and filled the map with thorns. Took a while, but very easy around level 30.
Frog - the second boss, but maybe even easier than the Spider. I blitzed this first try at level 34 or so.
Driller - this was a different story. The Driller was the hardest battle in the game so far - quite difficult to ensure zero losses. I was level 38 when I tried it and I failed to keep the losses down over 4-5 tries. I had read somewhere of the idea of using only 4 stacks (as that is the width of the map) so I used Paladins, Inquisitors, Red Dragons and Archmages. This was with 20,000 leadership, enough for 10 Red Dragons. I had a lot of trouble at first keeping the Inquisitors alive. I know they aren't a good choice for this fight, but I could use the Resurrection ability. Then I realised that the Driller likes to use his machine gun on the stack on the far side of the map (i.e. the Driller's right side). So I switched the Paladins to that side, and I did much better. My main tactic was to use Stone Skin and Magic Shield on the Red Dragons, who were doing about 3000 damage for a puny 110 retaliation. Spells cast were Healing, Stone Skin, and occasionally Haste. One or two Phantoms at the end.
So now I am level 39 with 21,000 leadership, and I am ready to smash Tekron. When I got there, I was surprised to see that the enemies were listed as Slightly Weaker, Equal in Strength, with the heroes Very Strong.
189 fights without loss now, and I expect to kick ass on Tekron and Umkas. Then there's Shettera, Nameless, Elon and Reha to go.
EDIT: one thing I am doing differently is NOT progressing down the Might skill tree very far. It seems to me that skills like Quick Draw, Nighttime Operations, Frenzy, Tactics and Power of Darkness are of limited value and therefore optional. Revenge and Bloodthirst are very nice, but maybe not a necessity. So I've decided to trade in a lot of Might runes for Magic and try to increase my mana pool and mana regeneration. It seems to me that the left side of the Magic tree is very, very useful. I want to max out Wisdom, Meditation, Concentration and even Linguistics, which will give me a large mana pool for boss fights. What do you guys think of this?