If you want, you could put Lissa and two or more characters in a corner somewhere and have the other characters remove their weapons. Just let the enemies hit your "defenders" while Lissa heals them. This may take a while though.
And have none of these enemies able to used ranged weapons.
Oh yeah. That too. Of course, if you have the Champions of Yore 1 DLC, it's much easier to just kill every enemy but Celica and just have her hit someone so that Lissa can heal them. You just need to make sure that your allies die.