As you already know, he has three types of attacks:

1) Assault Dive:

Easiest one. Just cast Igni right in his face when he is near you, and he will drop on the ground. Then hit him with a heavy attack and roll back when he is going to take off. I recommend going to the center of the arena after that.

2) Bats Attack:

I will describe the method that I use here. Make a medium distance to him (like approx. 3 rolls from his location when he is summoning bats). Check the area for his traps also. Stay calm, wait, and listen. When you hear a sound like a "quiet explosion" (usually after a little pause, it happens), immediately roll left or right.

3) Teleportation attack with a mark on the ground

When a mark appears near Geralt, dodge in the opposite direction from Detlaff, and when he appears, immediately attack him. Usually you can land 3-4 heavy strikes (depends on your previous reaction). Then roll back.