Some of the best combos you can use are in order of most deadly-to least useful.
1) The most deadly combo isn't a combo it is a weapon switch between a charged plasma pistol/battle rifle(carbine)
2) Second is the plasma rifle+smg combo, this combo is extremely good for sneak close range attacks because the plasma takes the shield out fast and the smg kills fast.
3) Third plasma pistol+smg combo this is good because the charged plasma takes out the shield and the smg finishes them off
(the next two combos require a little skill with aiming)
4) Fourth plasma pistol+magnum combo, this is good because if aimed right you can kill them in 2 shots of the human pistol because they have no shield due to the plasma pistol
5) Finally this combo is in my opinion the best it is at the least powerful part because it takes some skill to use it. The plasma rifle+magnum combo. This combo is so good because if you go automatic pr on them it takes the shield fairly fast and you can take them out with at least 2 shots.