Might not have to deal with him at all. That talk of "new employers" makes me think that Shimnir is recruitable. Plus he has a mugshot, which as any FE player knows signifies either a recruitable unit or a boss.
He's the only enemy with a mug. He's almost definitely the boss. And even if he was recruitable, we'd probably have to throw money/loot at him, something of which we are in short supply.
I dunno. The boss might show up later after the initial waves are dealt with. He's expressing doubt about what Prixima is doing, which means he might not have a very high price for people against her or can be convinced to wait for payment. After all, in FE the only ones to doubt that what they're doing is right are recruitables, recurring characters, or characters with some prior connection to a party member.
I'm not saying that there's no way he's the boss, I'm just saying that my sensors are pinging a little.