Merciless Onslaught is the 3rd stage in Axis of Evil.
This stage starts off with three relatively buffed One Horns. However, once you kill one of the One Horns a 4000% Assassin Bear will come out as the boss. You might as well just give up right there, since it's set as the boss, so it knocks your cats back and destroys your base in one shot. The Assassin Bear has 2200 0HP and 120,000 Damage. Very specific Shockwave Cats can defeat it, such as Cats in the Cradle True Form. If you somehow manage to kill the Assassin Bear, DO NOT waste any time killing the One Horns. If you take 33 seconds, another Assassin Bear will spawn and will continue to respawn infinitely every 33 seconds.
Use low damage knock-backing cats, such as Dread Pirate Catley or, even better, Brah Cats. As the enemy limit is 3 these cats can knock the 3 One Horns back behind the base without killing them, allowing you to destroy the base with no chance of an Assassin Bear destroying you.
Cat combos can help a lot, especially KB up, but also for the worker cat. This strategy becomes easier as the stars difficulty increases.
Sync One Horn together using meatshields. Use Awakened Bahamut once they are synced. Time with Maglev Cat and cat cannon. Use cat cannon and then use Maglev.
A simple and easy way to beat this level without gatcha is to use the Thunder Bolt base (needs to be level 4 or higher). First, use Manic Dark Cat and Manic Island Cat defeat one of the One Horns. Use Manic Eraser Cat to stall the One Horns near the enemy base. After one of the One Horns has been defeated (make sure it does the soul animation) count to 3 seconds and use the Thunder Bolt, the Assasin Bear should be frozen, use either Awakened Bahamut Cat or Maglev Cat to defeat it.
- The Assassin Bear that appears in this stage deals the second highest damage per hit in the game, 12,000 more than Cala Maria.