You can artificially increase the merc's skills in range and gadgetry offline (since all you do is run around in a custom SysLink match with no opposition).

To increase your Direct skill, you can put in merc bots and corner them -- sometimes they continuously run out a narrow dorrway you can exploit. By doing the Berserk attack (L3 - or click LEFTJOY) you slowly build the direct stat.

If you and five friends have 20 minutes to kill, set-up a private session and (by gentlemen's agreement -- not 'Vestibule' agreement) berserk and headbutt the spies to their final life before letting them run around stealthily for their stealth increase or just rapidly for athletics skill increase. Overall gain makes everyone happy.