The slow time ability can be very useful to aim to the head and make critic shots.
Soldiers are great for open combat situations, their only major problem is the lack of tech and biotic abilities. But you can solve this, keeping Teammates with these abilities.
What's up whit soldier?
First: Soldier class can use many kinds of weapons in the game, so you NEVER wont run out of ammo. Second: The special ability to slow the time can help you to aim to the head and make critic shots, this is very useful against fast enemies and numerous enemy forces. Third: The soldier's health is improved, with a good health regeneration rate and it can wear heavy armor.
Adrenaline Rush Concussive Shot Disruptor Ammo Incendiary Ammo Cryo Ammo Combat Mastery
So, if you like to go out there and shot down everything you can, soldier class is your choice.