From Brotato Wiki
Elite and Horde Waves
Elite Waves and Horde Waves are challenging waves that appears on higher Danger Levels.
When you are playing on Danger Level 0 and 1, no Elite or Horde Waves appear. When playing on Danger Level 2 and 3, one Horde or Elite Wave Appears And when playing on Danger Level 4 or 5, three Horde and/or Elite Waves appear.
The three Waves are spread across the later waves of the game. One Elite or Horde wave will appear on wave 11 or 12. If you're playing on Danger 2 or 3, this is the wave you'll face. The 2nd challenging wave will happen on wave 14 or 15.
The third wave is guaranteed to be an Elite Wave and happens on wave 17 or wave 18.
It is random whether you get a horde or an elite wave on the two first challenging waves. There is a 40% chance for the challenging wave to be a Horde Wave and 60% for it to be an Elite Wave. You can see the incoming waves in the bottom right corner of the screen in the Shop. (As seen on the Picture to the Right)
On Elite Waves, one of three possible Elites appear in addition the to other enemies of the wave. Each Elite has its own pattern of attacks and movement. If you manage to kill the Elite, a Legendary Loot Crate will drop, which gives a random Legendary Item, and also heals 100 HP when picked up.
Once an Elite has appeared, the same Elite cannot appear again in other Elite Waves.
The Silver Bullet Item can help you kill the elites, as it increases the damage you deal to them by 25%
On Endless Mode, 0-3 Elite Waves will happen for each 10 waves. However multiple Elites will spawn on each wave, and there is no limit for which elites can spawn. These Elites drop a regular Loot Crate on death. See Endless Mode for more information.
On Horde Waves, additional enemies spawn in addition to the regular enemies.
Enemies drop 35% less Materials during Hordes, but the additional enemies means that you typically earn more materials than on a regular wave.