All of the multiplayer achievements can be done in private matches with friends. Of course you can play it legit, but I can tell that you will need around 60+ hours to complete the weapon challenges and armor challenges alone, not to mention the miscellaneous achievements for it. Grab 2 friends and concentrate on the weapon challenges with them. You can do it with just 1 other person of course, but there are some challenges where you need multi kills. The best setup you can choose is "Seize Ground" with a point limit of 2500 and a time limit of 10mins (depending on how long you want to go, you can of course increase that number as well). The map you choose is not that important, but i found out that the one with the crashed Thunderhawks in the open is pretty well suited for around 95% of the challenges (should be "waste disposal"). Its not a full 100% because of the extreme range kills with the laser canon, the map is too small for those. As soon as everyone has a high enough level, the boosting works as follows:
The booster chooses whichever setup he wants and the other 2 players choose tactical with rapid deployment (unlocked on higher levels) and a second perk of choice. The booster is alone in his team, the other 2 are in the other team. Now the booster waits in front of the point closest to him, while the other 2 capture the same point. They get killed and choose the exact same point to spawn. With rapid deployment this takes only 3 seconds. So, with that said i will point out each achievement now. (you can do all of them of course with a guest/dummy account as well, but it will be way more boring that way)
This will come basically as one of the first achievements for obvious reasons. As soon as you reach a certain level you will unlock new classes and therefore the corresponding weapons. Since you will concentrate on the weapon challenges you should do it like that: As soon as it's your turn, choose the Tactical Marine (should be your first class anyway) and work on the kill streaks with the Bolter. A kill streak is when you kill 3 enemies in a row without dying. The problem is, if you now continue the streak and rack another 3 up, it won't count as a new streak. So, after your 3rd, get killed by one of the other players, rinse and repeat until your turn is over (I will go into detail on the challenges later on). The next is the Devastator class with the Heavy Bolter. This one has kill streaks as well, so do the same as with the one above. Last pick the Assault class, rack up Bolt Pistol and/or Chain Sword kills. Those only need kills, so kill as much as you want to. Achievement unlocks after the game ends.
You will get those almost naturally, but in case you want to play it safe, write down each game with each class you did. Do the weapon challenges class specific, therefore start with the Tactical (10 games with weapon challenges), then switch to the Devastator, etc. There are more than enough challenges with each class, so no worries you might do too many games.
Have 10 times more kills than deaths in a single game of Annihilation.
Now that you have a decent level, host an annihilation game, set it to around 20 kills and use a map of choice (preferably a small one). Now, get killed once and do the remaining 20. Do the same for the other players until everyone has the achievement.
This one will come almost naturally and if you want to you can of course keep track and count.
Now this one is a little tricky. You have to use the Vengeance Launcher, while the rest uses the Raptor class. Stick one of them with 2 grenades, let them jump up and detonate the grenades (LB) in midair. Rinse and repeat 50 times and the achievement is yours.
Kill 25 opponents who are capturing your Control Point in Seize Ground.
Capture a point and let an enemy get close to decap it (you have to stand far away for that because otherwise its contested and wont count). Kill him while he does it and dont let him cap it. Rinse and repeat 25 times and the achievement is yours.
Armor challenges are shown on the challenges option when you press Y. Those will come naturally since you get them by killing, winning and doing other stuff as either Chaos or Space Marine (each has its own achievements of course)
Fully customize a Space Marine and a Chaos Space Marine character.
This one is awarded for fully customizing a Chaos and Space Marine. You have to swap each piece of armor AND change each colour on every armor part. As soon as you leave the customize screen and go back to MP, the achievement pops. This will take some time in the game since you first have to unlock one piece from each armor. If you however have the preorder bonus skins, you can get this right when you start MP.
These are the reason why the game has such a high ratio and takes that long to complete. The weapon challenges are hard and time consuming and if you want to do them legit, good luck, because kill streaks and double kills are close to impossible. I will write here a list of the weapons and what you have to do with them (not the number, but the sort of kill you need).
Bolt Pistol: Kills
Plasma Pistol: Kills
Chain Sword: Kills
Chain Axe/Energy Axe: Kills
Energy Hammer/Energy Mace: Kills
Energy Sword: Kills (not needed for the achievement, additional when you played Warhammer kill team)
Bolter: Kills, Kill streaks
Storm Bolter/Double Bolter: Kills, Kill streaks
Stalker Bolter/Demon Eye Bolter: Kills, Headshots
Laser Cannon: Kills, Extreme range Kills
Vengeance Launcer: Kills, Double Kills
Plasma Thrower: Kills, Alternative fire kills (loaded kill)
Melter: Kills, Double Kills
Heavy Bolter: Kills, Killstreaks
Plasma Cannon: Kills, Double Kills
Get a Multiplayer character to Lvl 20.
Get a Multiplayer character to Lvl 30.
These will come naturally while doing the weapon challenges, so don't bother about those.
That concludes the multiplayer part of the game.
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