Unlike Campaign, Grimm Gauntlet is rather self-contained, despite having to be played online. Grimm Gauntlet is played on the same maps as Horde Mode. Apart from the lack of control centers, the maps behave similarly to the Horde Mode counterpart. Each wave Grimm are spawned, and when they are defeated the players advance to the next wave, receiving a Lien bonus, and some time will be added to the Onslaught Timer, resetting it if it was at 00:00. A group of 10 waves is called an Onslaught, and advancing Onslaught increases the number of Grimm spawned each wave and makes them stronger. If the Onslaught Timer reached 00:00, an Ursa Major is spawned.
The bonus Lien at the end of each wave is equal to how many waves total haven been completed multiplied by the number of seconds left on the Onslaught Timer, then rounded to the nearest 5. For example, finishing Onslaught 2: Wave 2 with 00:44 on the Onslaught Timer will get the player/s 12x44=528 rounded to 530 Lien.
Enemies have double HP in Onslaught 2, and triple HP in Onslaught 3 and onward.
Enemies deal more damage as Onslaughts advance, but it is currently bugged and only affects the host.
Unlike Campaign, if a player is not revived in the 10 seconds before their revive timer runs out they will not repsawn until the next wave and will enter spectator mode.
As the player defeats enemies, finishes waves, acquires Wave MVP (multiplayer only), Turret MVP (multiplayer only) or flawless waves (no damage to the control centers), they earn Lien, which can be spent on buying turrets. Turrets help the player/s defeat Grimm. Players can purchase either machine gun, icethrower, grenade launcher or shotgun turrets, which provide various advantages and disadvantages to the battlefield.
As the waves in an Onslaught go on they increase in difficulty, with more dangerous enemies spawning in later waves. Every 10 waves the cycle resets, so Onslaught 2: Wave 1 will spawn the same enemies Onslaught 1: Wave 1 did, just more of them that are stronger.
As the maps are based on Horde Mode maps, the Grimm will only spawn in the same places they would in the same wave in Horde Mode. For example, playing Grimm Gauntlet on the Emerald Forest map will have the Grimm spawning only in the first area, then the second and third areas only, then the fourth and first areas again. Or on the Mountain Glenn map, the Grimm will spawn by the starting area only for the first two waves, then everywhere after that, as the later areas are permanently blocked off.
Like Campaign, the more players are present, the more enemies spawn, therefore increasing the difficulty.
- Turrets kills do count towards Rank requirements, except Turret MVP does not count as Wave MVP.
- The maps behave very similarly to their Horde Mode counterparts, making strategy vary across the maps
- If the host disconnects the lobby will restart from the beginning.
- The mode is not technically endless: it stops spawning Grimm at Onslaught 143, but that's very unlikely to be reached without mods.