You started playing the game one day before official release. I hope you've successfully downloaded and started playing the full-release game now.
As for the demon commander, have your player character run (or dash if you just want to get it quickly over with) to the transponder (the fleshy blue thing you can interact with if you run past - or "up" on the screen).
He drops a neat flaming greatsword and loads of XP... if you can kill him, so here's how, in order of importance:
1. Either have a player character that can heal or have Shadowheart (cleric in pod that you can save) in your party and keep the MIND FLAYER healed and protected. Shadowheart has two 1st lvl spells she can cast in the fight. Use armor of faith on mind flayer, then...after demon commander has hit him two times, one heal spell.
2. Move a character past him and close to the transponder. More little demons spawn... don't kill them too quickly, because if you manage to kill commander AND imps BEFORE the two cambions arrive, the mind flayer insta kills your party.
3. Focus dps... don't use ANY spells except cantrips untill the commander fight. Use all dps...for mages and sorcerers, use magic missile, only on commander... when he is dead, grab his sword... and have all chats dash toward the transponder...then click it and fight ends.
It's tricky, and requires some luck... but the mind flayer can hit demon hard, plus stun if you keep mind flayer alive long enough.
Bonus: If you loot grease... throw in the two combions path when they arrive. Don't need to... just a little bonus thing you can do.
