In scene 1:44, It shows a marine firing one of hte Autocannons. HOWEVER, this is not the marine who fires the big gun, and not hte marine who downs the shivan.
How do I know? Because:
1. The marine with the big gun is in the left-front of the group, as is established earlier in the cutscene.
2. The marine in 1:44 is not this marine, because there is a Flashlight beam coming from the LEFT of him. Therefore, he can't be at the left of the group.
3. The Marine in 1:44 is not hte marine who takes out the shivan, because the marine who takes out the shivan a.has no flashlight coming from the left, and has the big gun muzzle, as is clear from the scene itself.
Therefore, we can conclude it was the Marine with the Big gun who took down the shivan
Case closed.