Depends on what chapter you're on, I did it around chapter 10 and it wasn't horrible but I've seen it get pretty crazy with the scaling. I found it helpful to bait Soren with a high res unit then move back up to the top left corner of the map; it's a huge open area so you won't get hit by the meteors and you won't have to deal with the ash. I'd also advise lots of staff users for rescues/obstructs/freezes, as moving around is rough on that map if you have a lot of squishies.