Fast answer
This mode rewards board stability, not just raw damage. Dewgrub, Buddi, and Voltfawn are excellent here because they keep doing useful work across long wave chains.
Horde Invasion is the main repeated-wave survival mode, where lane stability and wave control matter more than one clean burst window.
Horde Invasion unlocks as part of normal game progression and quickly becomes an important mode for testing whether your roster can handle repeated pressure.
This mode rewards board stability, not just raw damage. Dewgrub, Buddi, and Voltfawn are excellent here because they keep doing useful work across long wave chains.