Your AS is shrouded in mystery. To confirm my understanding of it, I checked your EVA in the same charsheet. The formula is supposed to be (EVA = 2*AS + LCK). Your LCK is 2, and your EVA is given as 3, indicating that for the purpose of calculating EVA, your AS is 1/2. That's still not 4 less than Oskar's, but some clarification from Haspen would be nice to have.
Edit: A potentially very stupid question. Can Arnold use his staff to heal himself? (No.)
Edit 2: Here's a plan (a kind of hybrid of darkpaladin109's and Criptfiend's) that brings back Siegfried next turn, keeps all vulns for emergencies and hopefully avoids a long slog. At least the first steps, up to and including Arnold healing Siegfried, can be carried out without knowing how mobile Oskar is, or whether he can double Gregory. What do you think?
--- Turn 5 ---
Friderick moves to [7,7], trying to draw Oskar's aggro. (This will trigger his PF but he can tank that javelin no prob anyway.)
Arnold moves to [7,8], ready to reach Siegfried next turn.
Gregory moves out of Oskar's range, [7,10] seems like a good choice.
Oskar does something, maybe. Hopefully he doesn't hit Arnold. Or crit again...
--- Turn 6 ---
Arnold moves to [7,6] or [8,5] and heals Siegfried, bringing him to at least 7 HP (Oskar-proof).
--- From then onwards ---
Friderick and Siegfried tank passively, hopefully keeping Oskar busy and contained.
Gregory zlorches Oskar whenever possible.
Arnold heals Friderick or Siegfried if necessary, otherwise he heals Gregory.
In case of an emergency, anyone can chip in with vulns.
NOTE: Arnold can take one hit from Oskar without falling, and is immune to crits, so he won't get KO'ed out of the blue.