1) On tactician difficulty, the commander has 195HP, and the time window for the player party to deal with him is limited to 6 turns (realistically), up to 15 turns (bottom line).
2) Shadowheart has 55% success rate to command him to drop his weapon and can do that twice for a total success rate slightly over 80%.
At least on Tactician difficulty, the best outcome, in my opinion, is this:
1) ensure everyone (2 temporary companions + Us) is alive in the end,
2) get the weapon,
3) ensure it is the player party that deals the killing blow to the mind flayer,
4) have the main character activate the "helm".
This way, nothing is stressful, the player will end up at level 2 as soon as gameplay control is returned to them again.