- Replacing your Swordman for a Witch is okay since your BT will later substitute the lack of physical damage.
- I wouldn't replace your monk since it's very essential in a party.
- You could replace your Viking, but only if you can handle the lack of Freezing Axe. Unless, you have some distaste of your Viking.
- You could replace your Artillery if you can live without Deep Insight and cross AoE.
If these were your only choices, I would personally substitute your swordman, spearman, or sniper, assuming your party can handle it. I recommend preparing a merc. room slot so you can swap mercenaries and see what will meet your preferences.