A strategy for the Soldier class in Mass Effect 2.
In this strategy, you will need a few specific guns and powers. First off, the idea of this strategy is that you, the soldier, do a lot of running and gunning, while you rely on your team mates for powers that deal a lot of damage. A power you will need for your soldier is Geth Sheild Boost or Barrier.
The squad mates you will need are Miranda and Tali. During combat, position your team so that they are in a well covered position. It is most effective to turn Auto-use powers off, as then your team will not waste their powers on weak opponents.
Rush up close towards your opponents with an assault rife or shotgun, using Miranda's Warp and Overload powers to deal with any enemies possessing Sheilds, Armor, or Barriers. Use Tali's Combat Drone or AI Hacking to distract any strong enemies, and attempt to get yourself into cover in a spot which gives you vision of most of the enemies.
Use your assault rife (Revanant is best for this) or shotgun (any shotgun you prefer) to deal as much damage as possible to all enemies within range. Use your Geth Sheild Boost or Barrier to replenish your shields and continue firing, all the while continually using Miranda and Tali's powers to deal with any potent attackers.
After eliminating most of the enemies, snipe any that are hiding in fortified positions and continue moving. When facing extremely powerful enemies, such as Mechs or Gunships, do the same strategy, except spread out your squad as much as possible in order to confuse the mechs and make them continually re-assess their targets.
If facing more than one mech, focus one down until its armor and sheilds are depleted, and then use AI Hacking to turn it against it's allies. After reducing both to only Health, you should have no problem dealing with them.
This strategy is effective in almost all situations, such as against Collectors, mercs, or mechs. Remember, the core of this strategy is continued damage from Shepard, supplemented by Miranda's high damage powers and Tali's sabotage powers.