for Skirmish, the more gun the better. Most sword mercenaries are useless for skirmishes.
here's my skirmish form :
but this form is kinda expensive xD
this is a cheaper form with your mercenary lineup :
replace monk with oracle for PvE
& Skirmish, unless you want to level him.
get a Sheriff or Janissary, both are great for PvE.
Recruit a Hwarang and maybe replace Minstrel with Empress (or Vampire), Minstrel has a really low damage and salvation can't resurrect dead mercenaries.
just do Alishan TBS 120 and Xiao Qiao's Escort or Flamethrower mission after you reach lvl 125.
these are the easiest and most profittable TBS at your lvl. StaffM + Witch + Oracle (+ Janissary for 3K & Alishan) will do great for TBS.
FB is still a must imo, it'll 1-shot mobs in skirmish and in TBS (except Africa) with maxed skill lvl + charm.
this is a skill set for farmer (non PvP)
80 FB, 70 PO, 1 Mana Shield, 1 Blessing of Life, and rest to Brutal Will.
and my skill set which is still good in all area imo (PvP-PvE-TBS).
80 FB, 70 PO, 1 Mana Shield, 1 Guard Dispel and rest to Blessing of Life.
both has its pro and con, so it's up to you to choose, but FB still have to be maxed.