Originally Posted by Morty
Last three attempts were a Human Infiltrator with a Mantis, a Krogan Soldier with a Scimitar and Widow and a Human Soldier with a Mattock.
Those aren't exactly the best class/weapon combos. There's nothing wrong with them, they're just not going to bring in amazing results. The Mantis is actually pretty good, since it's light, powerful and easily to get up to rank X, but it works best on a more caster-y character (power -> shoot -> power -> reload -> repeat).
If you want effectiveness, I've found that a Human Sentinel spec'd for rapid detonations is the most serviceable of the initial human classes. Beyond that, of the initial set of unlockable classes, Asari Adepts are still good and so are melee Krogan on bronze. Turians are decent, but want certain guns to make the most of their stability.