>>725706670
I like to just take the thief out turn 1 with Warp Ragnarok. Assuming you got at least bond level 3 with Celine in chapter 4, she can secure the kill. Place her in the top left deploy slot, and have someone ready to distract the nearby sword fighters. She'll tank the archer on enemy phase, but it won't kill her, then you can just vuln her on the next turn.