for airkill i go on rush on isla paradise i think its called , get in a copter and crash land into the mcom and usually get the kill!!! As for demo if you only need 2 kills you know what to do , smash some buildings up!!
Demolition man- This may take a while but this IS doable, what you need to do is use C4 and place 1 on each corner of the building and one on the ceiling. Rush is great for this because the defenders & attackers are always in the designated buildings.
E tu brute- A VERY easy way to do this is to go into a game and look in the scoreboard and message a couple of the enemies asking them to do the trophy, eventually one of them will respond and you can both take turns knifing each other. (Private match does NOT work)
Airkill- The great thing is you can use an UAV for this, just find a sniper go behind him and fly into him.