Now i've got all the new achievements, i thought i'd give some tips on how to get them.
Flag Dropped: We'll assume you're on Avalanche as thats the only legendary match with flags, your best chance of getting this achievement is by getting the rockets via the teleporter in your base, then going back to your base and camping next to the flag but just out of sight of anyone who comes to pick it up, then you can just one shot kill them when they pick it up.
Defend This: The best way to do this legtimately is to crouch through the teleporter in their base, kill anyone there and go back through the teleporter and stay crouched a little behidn where you come out so as soon as the other team run through the teleporter *BAM* assassination and achievement unlocked.
Alas, Poor Yorick: This isn't too hard as a majority of games just boost the achievement, but it's worth going in with a tactic, your best bet is to not go for it straight away, get a good power weapon like the rocket launcher and more importantly get the bubble shield, then kill off anyone currently gunning for the oddball and throw the bubble shield down over the oddball, so you can't be attacked from range and get your melee kills.
Look Both Ways: This achievement requires a great deal of patience, my friend and i both got sprees of 4 many times before finally getting the 5th. Avalanche is a great map to get it on as there are lots of targets all pre-occupied with flags/territories/bombs and there's a ghost. It can take over half the game to take this achievement as you will need to measure the chances of you getting a splatter and getting out without getting a kill before making a move, your best spots for easy splatters are the opponent's teleporter cliff, some people choose it to snipe/laser from meaning they aren't looking at the large red dot flying towards them on their radar. The other spot is the middle of the map where the shotgun spawns, just hiding out of sight down the hill then when the corridor fills up, boost through and out the other side.
Road Rage: This is a fairly straightforward achievement, invite a friend to your party and search for DLC big team slayer until you get avalanche, then just take it in turns driving each other in warthogs until you get your 5 kills.
The double kill and assassination achievements are simple enough not to warrant a guide, if you're really stuck get a friend/guest in your match and boost with them.