Halo: Reach

A Killionaire badge is earned by killing 10 enemies within 4 seconds of each other.

Pick a normal Firefight game, and grab a Target Locator from one of the Ordinance points. Wait for a Phantom to lower and as it does, target it. The Target locator will bomb the area as the Phantom drops the enemies easily nabbing you plenty of kills. I got a Killionaire the first time I tried this.

IGN made a video for easy Achievements in the Firefight mode. You can get about 9 Achievements in one game of Firefight.

Follow this video exactly and in about an hour an a half you will have 9 new cheevies including this one. =)

I highly recommend using the map Corvette and picking up the rocket launcher as it is way easier and faster to kill everything.

This one is actually pretty easy. Just make a gametype where you have infinite ammo without having to reload and you spawn with a rocket launcher. It also helps if you do 300% damage and they can only take 10%. Set all enemies to be grunts and play on the map Corvette.
Let all the grunts drop down from the ships bringing them in and start shooting. It might take a couple of tries since the grunts tend to split up into smaller groups.

For a good gametype, download this one: http://www.bungie.net/stats/reach/filedetails.aspx?fid=2007&...

All credits for the gametype goes to Rabidkitty4

I used the IGN variant and went for the million point achievement at the same time. Play on the Overlook map. When the drop ships drop troops on the far side of the bridge and river at the same time, head to the bridge. When the grunts start crossing, head over shooting. Kill all the grunts from that drop ship and then immediately turn right and kill the second group that are waiting. I managed 35 Killionaires doing this method while heading for the Million!

The Killionaire medal can be won be killing 10 enemies within 4 seconds of each other. The easiest time to get this is during the Bonus round, but it should be easy throughout the game with extra damage rockets and grunts clumping together. Use the following settings and you shall get the achievement in no time.

Set your Difficulty to Normal or above

Under Game Options, choose Firefight Settings and Wave Properties and change these settings for Round 1, 2 and 3 Round Properties. You have to change the settings for EACH ROUND.
*Now, for each Wave, Initial, Main and Boss, change these settings:
*Uses Dropship: Enabled (You will be able to hit clumps of enemies dropping from Phantoms for high scores)
*Squad Selection Order: Ordered
*All Squads 1-5: Grunts

After adjusting the settings and Skulls for ALL 3 ROUNDS, enter the Bonus Wave settings and make these changes.
*Bonus Wave Duration: 2 Minutes
*Uses Dropship: Enabled
*Turn on these skulls: Mythic, Catch, Tough Luck, Famine, Cloud, Tilt, Black Eye, Grunt Birthday, Cowbell, IWHBYD.
*Squad Selection Order: Random
*All Squads: Grunts

Back out of the Wave Properties menu and make these settings in the Game Options/Firefight Settings menu.
*Wave Limit: No Limit
*Turn Count: No Turns
*Time Limit: No Limit

You do not need to adjust the Custom Skull Settings. Back out to the Game Options Menu and choose Spartan Settings. Ignore the options here -- they won't matter -- and choose the Base Traits Menu.
*Choose Shields and Health and makes the following changes:
*Damage Resistance: Invulnerable
*Immune to Headshots: Enabled
*Immune to Assassination: Enabled

Back out to the Spartan Settings / Base Traits Menu and choose Weapons and Damage and make the following changes.
*Damage Modifier: 300%
*Melee Modifier: 300%
*Primary Weapon: Rocket Launcher
*Secondary Weapon: Fuel Rod
*Starting Equipment: Jet Pack
*Infinite Ammo: Bottomless Clip (Crucial: No reloading!)
*Equipment Usage: Unlimited

For Loadouts 1-6, change Hide Loadout to Hidden (this will ensure that you have the Rocket Launcher you set for your primary weapon above for the entire match)

I managed to get mine on the Courtyard firefight map.

This is easily done by yourself and within a few minutes of starting the round. As soon as the match starts use the sprint ability to run over to the target locator, which could be on either side, but for me it always appeared on the left side as you go out of the door. As the two Phantoms come down, target the one furthest away from you as it's landing, the ordinance should start blasting away as the enemies are jumping out. At the same time start shooting everything that comes out of the phantom closest to you (use grenades for good measure), and that should do the trick.

After reading several of the other solutions, which are all fine in their own right, I thought I'd share how I managed to pick this one up. It was actually pretty easy and I didn't have to enter any special settings.

My friend and I were playing on the Outpost map with the difficulty set on Normal. When the round started, I made a mad dash for the weapons drop, picked up the Target Locator, and ran back to the main bunker. I set myself up on the roof and targeted the first incoming Phantom (on the left side of the map while your facing out from the roof of the bunker). I managed to get the strike to occur just as it was dropping off it's first set of Grunts and other assorted troops. Wham, bam, achievement unlocked.

Good luck!

choose a normal firefight game then put it on Corvette then go to setting and put unlimted sets and give youre self invcibality turn on bottomless cup on so you never have to reload then give your self a class that has a rocket then put a round full of grunts then start it when all the grunts got on the ground go to the middle of the mapthe go crazy with the rocket

1.go into firefight lobby
2.put it on legendary(make sure it is rocket fight)
3.play until bonus round
4.when all the enemies all pile up,PULL THE TRIGGER OF THAT ROCKET LAUNCHER AND BOOM!!!!!!!!!!
achievement unlocked.
it worked for me on my other account

To get this without much trouble, use the IGN game variant, and grab the target locator, wait until the bonus round, and when you're getting mobbed, break out the locator and drop some pain on them.