A firefight would break out; pulse rifle rounds, smartgun rounds and shoulder cannon rounds being shot at each other. I think a predator can kill around 3 to 4 marines before being taken out. This would make a great action scene.
Hopefully this can happen in either AVP3 or in Predator 3.
EDIT:
Since the Predator is cloaked and jumping from tree to tree, if this was outdoors. The only way the Colonial Marines can see the Predator is from the green blood from getting shot, the flash from the shoulder cannon or by using a motion tracker, in which they see a dot on the tracker to know the predator is right in front of them.