Are Oshu particularly resistant to TM, or am I doing something wrong? With 190 weapons, I can kill level 2 Oshu easily. With 190 TM, I do literally 0 damage to level 1 Oshu, even on a full target. I've tried Bolt, STRA and Pyre, but none do any good.

how much mana are you using too?

They're resistant to piercing damage as well. Throwing spears at them does very little damage while throwing a hammer or axe hurts them badly.

From STRA's description it seems like it does piercing and electric damage.

You might be able to pick up a stone fist scroll from someone.