Killionaire

The Killionaire medal is awarded for killing ten opponents within four seconds of each other in standard multiplayer games (and Halo: Spartan Assault) and within less time in campaign and Firefight. In Halo 4 multiplayer and Spartan Ops the kills need only be within 4.5 seconds of each other.

Originally, the medal was represented by ten platinum/silver stars on an onyx/black hexagon. In Halo 4 the hexagon is red, rimmed with black. In Halo: Spartan Assault and Halo: Spartan Strike the medal is monochrome and imposed on a shield, and features a single giant star.

This is the most difficult multi-kill medal to achieve, as there are no multi-kill medals beyond this point. Also, it should be noted that you will get the Killing Frenzy medal after earning this if you don't already. The maximum time for getting this medal is 36 seconds (40.5 in Halo 4).

Halo 3 Multiplayer

 * When Bungie opened up the Grifball playlist on March 6, the medal became much easier to achieve.
 * This can also be more easily achieved through using Team Swords on Multi-Team.
 * Being the gunner of a Gauss Warthog.
 * Playing Swords on Small Maps such as Epitaph or Guardian, while having an Overshield.
 * Killing people with an Oddball while in a Bubble Shield or Regenerator.
 * Killing Zombies with a shotgun in the playlist, Living Dead. The best gametype to do this is Brains, due to the Zombies having no shields.
 * Playing in Team SWAT, due to the lack of shields.

Halo 3: ODST
In Firefight mode, getting multi-kill medals get progressively more points, the pinnacle being the Killionaire. Using these methods to get the medal help get more points, and as a result can help if trying to get the achievement for racking up 200,000 points on that particular level.
 * In Firefight mode for Halo 3: ODST, an easy way to get it is to use the Automag against a large group of grunts with the Cowbell skull and the Grunt Birthday Party skull, using the Automag to quickly score multiple headshots, and the explosion from the headshots, possibly killing other foes. That combined with sticking a Brute can gain the medal. This method is especially effective during bonus rounds.
 * At the end of a round, gather power weapons dropped by Chieftains, like gravity hammers and the Fuel Rod Gun. When the next round starts, use them liberally against the packs of Jackals and Grunts that drop from Phantoms. Because of the weapon's high power and the enemy's low health, this is a good way rack up lots of kills very quickly. This method is especially effective during bonus rounds.
 * Early in a round, using the turrets against the packs of Grunts and Jackals is an effective way to quickly eliminate them and get this medal. This method isn't as effective in bonus rounds due to the constant grenade throwing by enemies, and their higher health.
 * Using an automatic weapon to kill Drones is also a good way to rack up kills.

Halo: Reach

 * In Halo: Reach's standard Firefight mode, a very easy method of obtaining this medal is to wait until there is a mass of Covenant troops in one area of the map and then use the Target Locator to eliminate them and receive the medal.
 * In Firefight Arcade, the enemies spawn in groups; with some knowledge of the spawn locations and fast and accurate aiming in heavy-weapons modes (Rocketfight, FRGfight, Nadefight) it's possible to obliterate a large number of enemies at once.
 * In the campaign level ONI: Sword Base, it is possible to get an unlimited number of Killionares. Simply sprint from the very beginning of the level to the closed gate where the target locator is found, past down the incline. Avoid killing too many enemies. After picking up the Target Locator, target the enemies behind you on the ramp and move to the back of the area. If done correctly, one will receive a checkpoint right after arming the locator, and thus, it is possible to gain an unlimited number of Killionaire medals.
 * In the campaign level Long Night of Solace, one can earn a Killionaire halfway through the level by diving under the Ardent Prayer at the beginning of the second chapter and firing at the Banshees stored on the ship's underbelly.

Halo: Spartan Assault and Halo: Spartan Strike

 * Each additional kill within two seconds after earning a Killionaire will award another Killionaire medal.
 * In Halo: Spartan Assault, the medal can be earned most easily on Missions A-5, B-1, B-2, D-5 and F-5 as the player faces waves of enemies and has access to a Machinegun Turret.
 * In Halo: Spartan Strike, the medal can be earned most easily on Missions A-4, C-6, D-1, D-2 and E-5. As with Spartan Assault the Machinegun Turret is very useful for earning the medal; in fact, it is even easier as the turrets no longer overheat.

Trivia

 * In Halo 3 multiplayer, this medal does not repeat if the player kills 10+ opponents within four seconds of the previous kill. In Halo 3: ODST, it is possible to have more Killionaire medals than Killpocalyse medals (or any other below) as multi-kills only award a single multi-kill medal at a time. In all subsequent games, players will be awarded additional Killionaire medals for each multi-kill past 10. This means that players will most often have many more Killionaire medals than many other multi-kill medals in campaign, Firefight, and Spartan Ops.
 * This medal did not have an audio file in the Halo Reach Multiplayer Beta; as a result, the announcer did not shout "Killionaire!" when the medal is earned.
 * Earning a Killionaire in Halo: Reach Firefight will unlock the Crowd Control achievement.