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 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. The maximum time for getting this medal is 36 seconds (40.5 in Halo 4).
Halo 3 Multiplayer
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.
Halo: Spartan Assault and Halo: Spartan Strike