Is there any good strategy against the Geomancer without killing the Sappers?
It always feels disproportionately difficult, particularly compared to the Yog fight. If you don't kill the spawners, you will have a harder fight, but you won't get combo'd to death in a few turns from bad luck. I was at ~40 hp before my last move.
All of the geomancer attacks are telegraphed, if you mash the attack button he'll get ya for good. Do a hit check for the next attack of the geomancer. The rockfall almost always has a place you can stand.to evade the attack. The rockslide can hit you if you are in a tightly position so destroying some rocks before getting close. And clear the map before starting the fight other mofos can come and fuck ya.
I mean, I know they are choreographed; that's not the issue. The question here is specifically about not killing the Sappers ahead of time so you can get the 'complete' fight, so to speak. When you have rockslides on consecutive turns or even two on the same turn, it's nigh impossible to avoid getting hit (sometimes literally).
I prefer the getting the Geomancer since it can clear out the mine for me and save some time(hunger) having to mine, as well as find some of the rooms.
Once I spot it, I'll engage it right away. Clearing any mobs in the immediate vicinity first of course. If gnolls turn up during the fight, I'll take them out before reengaging. The geomancer's attack has a range so I'll lure any guards out of sight so the fight is easier.
But I don't bother clearing any rocks during the entire fight. Even with a sapper nearby. Typically, you can dodge either of his attacks, as well as the sapper's. Sometimes that's not the case. But I'll charge in and take the unlucky hits even without a backup, like chains or the hourglass. I play with 5 challenges and usually I'm not pressed for resources that I can spend the time to clear out rocks during the fight, but maybe taking a hit or two from the geomancer is an acceptable alternative. I think with more experience, you'll find the geomancer ain't so bad.
Be wary of standing by pitfall traps since the rock throw attacks can activate it.
I won't go down the dungeon if my armor is lacking. If I have mail+2 or better than I'm good to go. Maybe leather+3.
Buffing your move speed and attack speed surely makes it easier to fight while not getting caught by the rock attacks. Perhaps you would also look into damage resistance actions like Iron Stomach or the stasis from the Hourglass?
i haven't tried doing geomancer without killing the sappers first, but blink stones and haste potions (and/or their exotic) could be a big help in getting out of the way quick. a scroll of affection or rage might be good too to distract the minions from hitting you. you might want to save the bees for the actual bosses but they could probably work here too on the minions.
Seems like something you'll only be able to do with the right equipment. Maybe try stocking up on haste potions or shrouded fog, or have an artifact like the hourglass or chains to help avoid the rocks better.