Here's how I would do it.

Balance the encounter for JUST the PCs. The Level 2 Artillery, the Level 2 Skirmisher, and a couple of minions.

For the allied NPCs, give them more enemies to fight "off-screen" while your PCs focus on their main problem. Probably the captain of the guard says, "My men and I will take care of these wolves and protect to townsfolk - you go deal with that witch!"