Spoiler
- if there's a single Materialiser, he tends to move to a side of the square (he doesn't stay in the centre) so I simply put my character on the other side and he can't hit me
- if there's a group of them, they usually don't cast their fireballs and other spells in a "salvo" (all at one time), but independently on each other, so I move my character to face the Materialiser that has just casted the fireball and I do it everytime any of them shoots
Plus of course I've got a few MON VI potions at hand - with my current HP one Materialiser's fireball still hurts me badly but can't kill me.