A good split between 3 high damage troopers (maxed snipers) and 3 tanks (HP regen + shotguns) usually will get the job done and also allow you to beat levels where the enemy is 10+ levels higher in training.
This is correct. Sometimes you will have to adjust to have 4 (or even up to 5) high damage troopers, depending on the enemy's troop composition. After several games, you should have a feel on when to make those adjustments.
One thing I'd like to point out, is the use of disablers (Stun/Blindness/Demoralization). Try to ALWAYS check the items equipped, and see if more than half of the enemy is vulnerable to those disablers. Sometimes the proper use of disablers is the ONLY way to beat a level.
- Stun. Renders a target unable to move or attack. The duration depends on the weapon used. Stunning at least 3 enemies almost always ensures victory. Be sure to equip your close combat troopers with Anti-stun equipment (BKB or Driller's Helmet) to avoid being stunned by friendly fire.
- Blindness. Reduces target's accuracy by 90 for 4 seconds (Flashbang). This effect is HUGE especially if the acuracy can be brought down to ZERO. Note that Accuracy affects both critical rate AND the damage output. Successfully landing the debuff is sometimes the same as simply stunning the target for 4 seconds. This is useful against Snipers and Rocket guys.
- Demoralization. Stops regeneration and decreases armor. Most levels are almost impossible to beat without this debuff. The regeneration of some of the AI troopers are too much. Also, the armor reduction is always nice to have.
These tips are also applicable for PVP battles.