LEGIT-PLAY METHOD
Buy a sniper rifle whenever you can.
Dig Site is an awful map to try for this.
Dwarfs cannot be killed with a single headshot. If you're sniping, you're still for a while. Get what I'm hinting at? Be a dwarf to improve your own lifespan whilst sniping.
Also this means that you'll need multiple shots to take down enemy dwarves.
Smartlink will give you an extra level of zoom.
Crouching and smartlink both improve your accuracy.
BOOSTING METHOD (works for all get xx kills achievements)
If you don't have 8 people for the session, don't even try. Nothing will count, nobody will get an achievement. The optimal number is 13 people. For this guide I will assume you have 13 people, but if your numbers are different it's easy enough to figure out :)
Create a private game. Invite everyone in. The maximum party size is 8 and Windows players won't be able to join [the party], so tell everyone the order of who takes their turn and when at the start to ease confusion later.
Have 5 people on RNA (either the first 5 to take their turn, or people that need the achievement for winning) and the other 8 on Lineage. You won't get an achievement if there is less than 4 people on either team, and having a fifth helps in case someone disconnects.
Set the map to Dig Site (on Raid).
Host makes sure Killer is on RNA and RNA has 5 players, then launches again.
The entire Lineage team should stand still (NOT even running around).
Everyone on RNA gives their money to 1 person (from now on named 'Killer').
Killer then buys a weapon they need as well as smartlink and either smoke or glider.
As soon as the round starts, Killer jumps down the well by the RNA spawn and uses glider/smoke to stop them taking damage at the bottom.
Killer then realises he is at the Lineage spawn zone and procedes
to kill the entire team. Round finishes.
Killer repeats this a further 5 times to end the match (RNA wins 6-0).
The 'Killer' title then passes onto the next player.
Host makes sure Killer is on RNA and RNA has 5 players, then launches again.
If done correctly, each time it's your turn you get 48 kills (8 players * 6 rounds). You will also pick up the Godlike achievement if you don't take that fall damage. If everyone agrees, you can also get Unstoppable if a few revives are performed on the Lineage side (other than that, there should be no revives).
If Killer intends to go for the Special Delivery or Master Blaster achievements then the entire Lineage team should group up.
If Killer intends to go for the Summoner achievement, Lineage players should be any race other than dwarf. Elf is preferable.
Whilst the Lineage team just stands around doing nothing (so they can be easily killed), the RNA team can do whatever they want to (EXCEPT following Killer to the Lineage team).
It's an ideal time to work on Heavy Smoker/Teleporter. In my boosting sessions we ended up with the RNA people running around the map in a race- as the rounds progress we used teleport/wired reflexes to go faster or strangle to hold up the other racers :) And sometimes we all just summoned Minions and had 'Minion Fights'. It was that or standing around doing nothing.
This boosting method works for all weapon achievements, as well as Master Ninja/Target Lock/3-for-the-price-of-1/Summoner.