How jumping in mid-air works
If you simultaneously press jump, up and M (the map key) in mid-air without having a map in your inventory, you will do a jump.
If you hold all these keys down, you get stuck in the beginning of the jumping animation, which is sometimes helpful and looks like you are sinking into the ground.
The map key also makes you not receive fall damage.
The general route
To end this game, you need to banish the Sleeper. To do that, you must collect the swords from each of his five priests (after killing them). The way into the dungeon is supposed to be blocked until late in the game, but there are several ways past the barrier. The only detour in this run is to open a chest, which gives me some magic scrolls, which help me pass some barriers in the dungeon faster.
The 1st priest (Ball Lightning)
The skeletons in this room are by default hostile to the priest. So you only need to let them live and have them attack him instead of you to win the fight without doing anything. I save and reload to reset the aggressiveness of the skeletons and turn the switch to summon the priest. When he attacks me, I sink into the ground to dodge his attack and let the skeletons do the work.
The 2nd priest (Chain Lightning)
This priest will (unless you are not too close to him) approach you before he draws his weapon. If you save and reload at the right point of the weapon drawing animation, he will draw his staff, but use it as a melee weapon instead of doing magic.
I then jump past a trigger that would have lowered a gate and lure him in his own trap. The trap does not do enough damage to kill him immediately, but the damage is reapplied every time I load.
At the end I jump past the trigger again. Would I touch the trigger, I would have to go down to the trap and activate a switch.
The 3rd priest (Pyrokinesis)
This is imho the most impressive part of the run (if you know what is going on). The priest kills you almost immediately, when you get into his range with no chance of survival whatsoever at level 0. Previous speedruns would lure a monster near him that would knock you unconscious until the priest walks out of range. Here is do a jump so I end up in range, but not in sight. I then do a jump into his sight and save and load in midair. This way I skip him walking up to me.
Now I sink into the ground, which for some reason makes him charge forward blindly. I can run past him and save and load to make him stop running. On the altar in the middle of the room, there is a spot, where a pillar descending from the ceiling will kill you, if you stand there too long.
I go into a corner and sink into the ground again to make him charge forward. When he is under the pillar, I save and load to make him stop running and get killed by the pillar.
The 4th priest (Storm of Fire)
I use a Fire Bolt scroll on the priest that does no damage, but lures him into his own trap. Then I activate the trap and kill him.
The 5th priest (Breath of Death)
This priest does not immediately attack you, but instead just warns you to leave since you're weapons are too weak to harm him. I get him to fall into a pit and die.
The final "fight"
Before going into the fight I load an older save, get into a dialogue and open the menu at the same time. When I load a save now, I am still in dialogue mode and nobody can attack me or talk to me. So the "fight" is just walking from one shrine to the next.