This is my recommendation. I notice you don't have a Viking, get one. Gunner and Archer is eventually dropped unless you want to keep the Archer for the eventual Dark Archer. These recommendations are for PvE only naturally. I play the game for PvE and only dabble in PvP.
Midgame PvE: (lvl40-70)
Yes, spear merc needed to pair up for dual lightning spear on a single column. Total immobilization of said column after both are lvl80+ and upgraded. The utility of spear is it's ability to totally deaden a column. This is especially useful for Boss fights at this level. Gunner is dropped around this point due to being pretty much useless and taking up room. Archer is dropped for the INFINITELY more useful Prophet.
High game PvE (80-100):
With lvl80 and Guardian stones, as well as decent lvl80 VIT rings (crucial) the Viking can now frontline. Spear merc gets moved to midline with the spear main and the arty is dropped for cannoneer. Shaman is dropped for Oracle, and the new midline now becomes a VERY damaging Seth's Will line. Only weakness of this formation is the lack of ranged mercs. With the Lady Knight and Janissary most likely at very low levels by this time, they do not yet fit into the formation. Around this point in the game is where PvE boss fights are largely made easier by the use of Seth's Will on the midline. Arty is dropped due to it becoming increasingly squishier at this point due to the increased attack on enemies, and the fact that Deep Insight's -DEF value has already lost it's worth. The only reason to use it now is for the free health check. Frankly, that's not good enough a reason to not swap him for a Cannoneer, who's Smoke Bomb DoT is superior and causes enemies to miss more, thus increasing your survivability.
Endgame PvE (100+):
Now with the inclusion of the strongest damager, the Janissary, and the best tank, the Lady Knight. This is pretty much the formation that will take you to endgame. The Viking also is upgraded to Northern Viking and gains in stats as well as the ridiculously good frontline murdering Ice Ridge Axe. By this point the 2nd spear merc is more of a hindrance and is no longer needed. You will be using less and less magic spam on Boss fights and the game becomes more about having bigger and more higher +'d whacksticks. As such, keeping a spear merc for the use of Lightning Spear is a waste... Why disable it? KILL IT! Cannoneer and Prophet can be rotated if ranged seth damage is needed, front line can additionally be Seth'd for melee damage. Recommend to swap the position of the main with the BT before Seth'ing as a seth'd spear main does MARKEDLY more damage.
Final Formation (110-120+):
Jani/SpearM/Pirate or Cannoneer
Optional but recommended if you can get them, for PvE purposes, the Pirate and Spartan combo is THE ultimate Boss killing combination. The addition of Pirate makes Lightning Spear useful again as the Pirate's Toxic Sword is the ultimate AP burning skill. Combine this with Lightning Spear, and the Spartan's War Cry, and every single boss you fight will probably get 1 move every 2-3 turns, and when it does move it'll only swing at you once, harmless. A boss under war cry, regardless of how big it's power is, once you get the fight down to 1v1, is the most harmless thing. A boss under Toxic Sword+War Cry is not even worth giving any attention, just Seth your front and midlines and go watch TV.
EDIT: Additional possibilities for Endgame PvE.
The 'OMG Heck Less Prz!' Formation:
Only doable if your pockets are extremely deep and you have +10 armor on the main, spartan and pirate, as well as +10 rings or dangerously close to. +10 weapons go without saying. If you are that well geared, this gives you arguably the strongest possible melee seth line, as well as the strongest possible range seth line. You will be able to massacre anything. NViking can GTFO at this point, he's a liability with his slow AP gain. Frontlining the spear main is usually a Really Really Bad Idea(tm), but if you are extremely well geared, you can do it. Likewise for a Pirate, +10 armor is almost MANDATORY to frontline her as she is an EXTREMELY squishy melee. Addition of Elementalist to the back line makes the expansion endgame content (Aztec onwards) a total cakewalk. She totally DESTROYS their formations before you even get a shot. But for many, this formation is nothing but a dream within a dream to achieve unless you invest EXTREME amounts of time, or spend a lot of real money in order to gear up.
Future Dual-Seth Formation:
Somewhat goes without saying... similar to the above, but the addition of the Chainsaw merc (Punisher) and the Hwarang to increase range multi rate makes this a ridiculously damaging combination. Again, similar to above, Pirate needs to be VERY well geared to stand in front.
Why? He is just too damn hekk is why. Note: put the Pho at the back. The front and midlines are taken up by his weapon when he removes his pants.