I propose that the following is one of the most well rounded teams for dungeon exploration.

Team Member 1 (The Runner):

Weapons:
Undecided

Shield:
Omega Shield

Armor:
I've heard of a Quicksilver set that boosts Movement speed and if so then I'd definitely get that. Otherwise I'm undecided here.

Helmet:
Quicksilver or undecided

Strategy:
-To ball up the enemies by running around and around them and then leading them to the other members.
-To disarm spike traps and to generally figure out puzzles so the team progresses safely. (For instance in one of the Fiend levels there is a spike trap room that becomes quite dangerous if multiple people attempt to do it at once as they spike each other accidentally.)
-To block incoming fire for the other members of the team, specifically the bomber and other non-melee

- - - - -

Team Member 2 (The Bomber):

Weapons:
Graviton Vortex

Shield:
Undecided

Armor and Helmet:
Any demo set preferably attempting to decrease charge time by as much as possible.

Strategy:
-Vortex the enemies that the Runner pulls to him

- - - - -

Team Member 3 (The Sword):

Weapons:
*The sword which can best take advantage of Graviton Vortex, likely a 360 charge attack sword

Shield:
Undecided

Armor and Helmet:
Primary focus on boosting up the DPS from that charge attack

Strategy:
-Spin attack the enemies the Bomber traps to deal mass damage to multiples
-Quick elimination of single targets

- - - - -

Team Member 4 (The Gunner):

Weapons:
A plethora of guns so the right damage can hit the right enemies.

Shield:
Undecided

Armor and Helmet:
Primary focus on boosting ranged DPS to eliminate stragglers.

Strategy:
-Fast gate/switch opener
-Distracting healers and other prime targets (perhaps Shock weapons would be good)
-Finishing off targets

- - - - -

I could see this team handling most situations except perhaps for some bosses if they require a significant amount of dps within a short time. The Runner will always split off from the group if the path diverges to check if the route is worth going down or if it's just treasure. I realize an 'optimal' team in a general sense is kind of contradictory but I believe this is the most well-rounded team at the very least, given that the players can fulfill their roles.