The Invincible Medal is a medal that you gain for killing opponents in a spree and considered one of the most toughest medal to gain. In Halo 3 and Halo: Reach Multiplayer, you can get this medal by killing 30 opponents in a row without dieing. On the other hand, it takes 100 kills in a row to get this in Halo 3: ODST.
You might easily get this medal by trying to compete with beginners or people who sucks at it.
Related Threadsneed help with invincible centinal on normal mode on 360 - last post @ May 2, 2015
The Invincibles Discussion (spoilers) Raid boss baby! - last post by @ Jul 17, 2014
terramorphous the invincible - last post by @ Jun 12, 2016
The New Invincibles Are...Regular Enemies - last post by @ Apr 2, 2013
Invincible quests disappear - last post by @ Jun 29, 2013