0 day Attack on Earth

This will require you to be on co-op mode, but take note that it requires you to be online. And you will need to kill 100 orange targeted enemies (the other ones do not count towards this achievement). But getting this is very hard, let alone a very few people who are attempting to getting a full 200G or less each year!

If you're going for this achievement (along with Like taking candy from a baby), you might want to find a person who hasn't got this yet (either here or 360A); if found a boosting partner, take turns and share the kills. However, if you can't find someone, let me know by PM here or XBL.

Update: If you happened to scored some orange targeted kills but you have been disconnected due to lag, your stats will be saved. The leaderboards are rather bugged, making it hard to tell what's the actual kills you have. In other words, just keep playing until the achievement pops.

You have to kill 100 of the largest enemies (the orange ones in the rader), the others don't count towars the total.

After getting the achievement out of the way, I will let you know how I did this in just one afternoon.

You will need a boosting partner or two Xbox 360 with two profiles connected to X.Live (This is my case). And kill 100 Orange targets, the bosses on each day.

Create the Multiplayer Co-Op Game in NEW YORK, with Friendly fire OFF and 1 private Slot (so you can Invite your boosting partner) and start de Game.

You have to take the X-B Aircraft or X-A Aircraft wich deals a lot of damage on those orange Bosses. And get ready for action. These enemies, have 3 kind of tentacles with 3 diferents attacks. So what you want to do on the beggining is get your weapon to it's maximum power. Than go for those ungreatfull enemies.
Two strategies:
1st Stay in a long range from them, close enough to harm them but far enough to don't activate the STONE FIRE Attack and the FLAMEFIRE. Just look at the small tentacles, they bounce 2-3 times before they try to attack you, when they do, that's the moment to attack the tentacle and get it out of the way taking a lot of damage to the monster.
2nd Get as close as posible, when the monster has fully arised, Use the bomb as it will eliminate half his tentacles giving you space to be very close and giving the maximum damage for each shot. (This is my strategy)

Try to get every enemy from day 1 to day 6 and let yourself get kill in the final day with the final Boss, unless you have plenty of life and you're willing to give him a rest. Without taking the boss, You will get 19 Kills per play in just 16 minutes (that's my time) so, 100 kills in a lil' more that hour and a Half betwen loading screens and creating match.

Deaths are like this, each number representing a day:
1-3-3-4-3-5-1B

TIPS:
- on low HEALTH Relay on MOSQUITO, they are very tedious, but give you a good boost of health and can be kill in one shot with the X-B Aircraft
- If you're playing alone with a second Xbox, just watch your partner getting absorbed for the mosquito or killed by the cage yellow enemy (can't remember the name), as your lives are his lives too.
- USE Every bomb in each day, they reload for the next one.

Hope this helps and get a lil' more info on the previous guides.

NOW GO GET THESE FUC..NG ACHIEVEMENTS!!!

So I'm just going to get to the point. This achievement is known to be glitchy.

In order to get it, you have to defeat 100 Boss Monsters whilst playing online Co-Op.
The Boss Monsters are the ones that have an Orange Target indicated on your screen, to where they are known to be hiding. The amount of Boss Monsters you can get depends upon the level your playing in. As you progress in the campaign you are doing, more Boss Monsters will appear, giving you more kill progression towards this related achievement.

You can have up to 4 people playing Co-Op with you at the same time. But only one person will claim a kill when the final shot on the monster is a direct hit on them. For that reason I recommend finding a partner to help you out with the game, or just bring in a dummy account and keep them idle throughout the duration. I pearsonally chose the latter as the game is Backward Compatible on the XBox One and I used the Share With Gold feature to get my dummy account to boost the game with me.

Among all the campaigns you can choose from to play Co-Op, I recommend sticking with "New York", as that has the easiest Boss Monsters to defeat. Tokyo has a Monster that has 2 Stages before it can be defeated, which alone can be a daunting monster to kill (it is kind of a robot). And Paris has a Monster that can only be shot in one area on it's body (giving more work for the user).

At times, whilst you play the campaign in Co-Op, you may notice on your screen a pop-up will say "Help". This means you, or one of your partners is in trouble, so if you can, try and find them to assist their needs in getting rid of a monster, as leaving them to suffer will cost a life and then they may not be able to respawn back into the game with you, until you have lost all your lives, or the level is complete.
If you are playing this with a dummy account, consider seeing that "Help" word quite a few times, late into the campaign.

You can check your progress by going to Leaderboards, then scrolling to find "Most Kills".
Now here is why I say the achievement is glitchy. According to the Leaderboard, when I had 100 Kills, I didn't get the achievement. I ended up getting the achievement at 133 Kills. The only thing I done differently was test the Tokyo & Paris Campaigns once to see if they we're easier to beat than New York. There I killed the Boss Monsters a few times that we're lurking about. These monsters that I killed must have not counted towards the achievement, because I believe once you have picked a certain Boss Monster to kill for the first time, that's the only one that would count towards the achievement. Killing the others won't count towards the achievement afterwards.

In the end I played a few more campaigns in New York afterwards and I finally got the achievement when I got 133 Kills claimed on the "Most Kills" Leaderboard.

You will be looking at around 2 - 4 hours of playing the same campaign over and over again before one of you will be getting this achievement (doing it with human partners in Co-Op). I hope you have some time and patience when you decide to go for this achievement.

Good Luck.