Well, My guess would be him not knowing the current objective of the mission. I did it once where I forgot about totally capturing a cp, and took me two minutes to figure out. Its good that the computer really sucks at killing each other (and I whacked up god knows how many kills for that)
For OP, just make sure you COMPLETE YOUR OPJECTIVE rather than killing everyone you see. They are infinite so don't borther. If you want medls play in freeplay and find a right map for the medals you want. Oh if you don't like it, just SWITCH OF FRIENDLY FIRE. It feels much better, since you don't hav to worry about stupid (so-called friendly) AI getting killed by your own attacks.
Really, the only hard level would be Hoth. The rest is easy.