Using the time dilation, rifles, and dash of the soldier class to maximum effectiveness.
Of course, different enemies require slightly different tactical choices, especially Boss enemies, but overall the most effective use of the Soldier's unique ability to dilate time is in using it to line up a devastating shot (or series of shots depending on what rifle you are using) and then quickly duck down again before anything can happen.
That may be pretty self-evident, you'd do that anyway regardless of time dilation, but with time slowed down for you, you can use it to pick off the more annoying, and dangerous, enemies first, such as the pyro enemies, scions, and heavy mechs, with a well placed headshot from your sniper rifle. As the game itself says in loading screens, headshots usually cause superior damage and there are plenty of upgrades to increase headshot damage as well.
Before you take the shot, you should bring up the powers wheel to pause the action and then move the camera around so that your crosshair will be close to the enemy you want to target (you can use the wheel to scout out enemy waves as they appear), reducing the time wasted to get the crosshair directly to the enemy head in the time dilation mode. You might want to also use any ally powers to weaken the enemy and active an appropriate ammo power before you take the headshot, hopefully killing the enemy outright after your shot.
If it survives (on insanity that's usually the case) you'll probably be out of time in the time dialtion mode after your sniper shot and since sniper rifles are slow to reload, quickly switch to your assault rifle to finish the job. This should prove effective for almost every enemy, even in insanity playthroughs.
Obviously, if your shields have been taking some damage while you were lining up your sniper shot, it may be more prudent to simply duck and recover, and wait for your shields to recover first.
Also note that the time dilation takes a very long time to reactivate, closing all other powers to you in the recovery time, so be aware of your surroundings before you use it, or you may be bemoaning your situation when the use of another power would have been more helpful.
A second important use of the time dilation is not nearly as commonly used, but very important when facing troublesome boss enemies who either themselves or have lackey come straight toward you. If an enemy is very close you can use the time dilation to in effect increase your dash speed and retreat to safer ground with minimal risk and damage. Against heavily armored enemies who relentlessly chase after you, this is a good key to victory.