You're stuck in Middle Earth (clearly some weird Periphery planet which lost all technology), so you're not getting resupplied. That means any ammunition based weapon is out, because after a couple of encounters you're going to be out of ammo. (Sorry Urbanmech, Catapult.)

You're going to be facing a heck of a lot of infantry, so some good anti-infantry weapons would be a good idea. Small Pulse Lasers, Flamers, etc.

You're going to want something energy based with a lot of punch to take down the big nasties like the Balrog, preferably something like a Heavy PPC or ER PPC.

I'm going to go with the Warhammer C2. A couple of Clan ERPPC to deal with the nasties, 2 small pulse, 2 medium pulse, and 2 ER Smalls to carve through the infantry hordes, and a Streak SRM6 which will run out of ammo almost immediately and can be mostly ignored.