Knock Minthara out with non-lethal attacks at the Goblin Camp
Do not kill her
Make sure she is temporarily hostile, not permanently dead
Finish the Goblin Camp without siding with the goblins
Proceed to Act 2
Find Minthara at Moonrise Towers
Free her from the prison area
Recruit her during the Moonrise escape or after the confrontation
Keep her alive through the rest of the game