Heal all strikes me as a worse spell to have memorized than the regular heal, because it doesn't reset nearly as quickly. When you're really in the thick of things, you can't wait another 30 seconds for your next heal. It's better on the pet, IMO. But I don't know if you have both heals memmed. I've got heal on myself and heal all on my pet.
On "very hard" level, I got through the end boss with a vanquisher, which wasn't too hard, and with an alchemist who specialiized in summoning and had very little else besides a maxed charm mastery and critical hits, which was easy. But he was overpowered for the dungeon at the time because of playing maps. I don't look forward to trying it with a destroyer, though, especially if I don't fix him up with anything much in the way of summons.
I'm pretty sold on having the zombie-summoning spell on the pet. He summons them a lot, and they serve as distractions. I like them on my hero, too, because then you have six other things to run your enemies through. And your hero's own set of zombies will heal him a little bit every time they hit the enemy.
One last suggestion -- I fought my last hero, the summoner-alchemist, through about level 51 or 52 entirely on maps. They tend to give you at least a couple of named bosses on their second levels, which helps you acquire fame very quickly. That gets you extra skill levels that beefs up your character considerably, while at the same time the character gets a little stronger compared to the story dungeon.
I combined that with putting as many points as I could into Adventurer as early as possible, increasing my fame -- and general experience -- that much more. I think I had a fame of 39 at level 48, for instance. Fame (and extra skill point levels) comes much more slowly in the story dungeon, as does loot.
However, I've found purchased maps harder, since everything is always right at your level, bosses are more often clustered right around the entrance, and if you die, the map is over unless you pay what's usually a very stiff fee to resurrect inside the map.
It's a much harder way to play, but you come out stronger. If you finally feel like going back to the story dungeon, it will be easy and only very slowly get any harder (or any better loot for your now too-advanced character). It's still fun in its own way, and a relief from the intensity of playing maps. Your fame per level will go down though, and probably never catch up. But that's okay. You'll have had plenty of extra skill points from getting so much fame so early.
So if you don't like the final boss fight, I'm not arguing that. Just mentioning there are ways to make your character a lot stronger, so the boss doesn't seem so tough anymore.
