How about a Merc who has Matching Machine Pistols for both primary and secondary weapons. He can dual wield them like the action heroes in John Woo movies.
Perhaps with a stick 3 jump ability to make his parkour even more reliable. Or better yet a slide on the knees forward ability to rush the last bit of distance to an enemy (not to mention hopefully throwing their aim off).
You could even allow the Merc to choose left hand, right hand or as the class special dual wield.
If machine pistols are too much, how about 2 burst fire pistols with extended mags in them. Glock 18c’s being an example. Call em “Schlock’s”
And name the Merc Woo while your at it 

I like it.