After completing the first Base Defense in the end game section, a "Powerful Creature Detected" special mission type will appear. They are difficult bosses to defeat, but you will get orange loot boxes for defeating them. It is recommended to first unlock the new character classes and put points into them before fighting the powerful creatures. The following are some of the powerful creatures you can fight and how to defeat them:

Big Mouth: The first boss is Level 43. It is a huge T-Rex monster with a large mouth and a side-mounted mini-gun that can quickly kill you without the correct equipment. Grenades do not damage this boss. To make the fight easier, use the Scout's invisibility ability. Hide behind the fence entrance into the ruins arena, as Big Mouth cannot follow you. Bring a powerful shotgun, heavy arrows, C4, turrets, grenades, and molotovs. When your invisibility drains, run and hide. Wait for it to come back, then repeat the process until it is defeated.

Frostbite: The second boss is Level 50 and only appears after defeating Big Mouth. Frostbite is a flying octopus-like monster that is actually quite a bit easier to defeat than Big Mouth. Use the Scout's invisibility ability to clear the arena of wanderers, then shoot the weakpoint with a sniper rifle. When Frostbite glows orange, shoot the spot with a powerful sniper rifle. Hit that spot twice to stagger Frostbite, knocking it onto the ground and stunning it. While Frostbite is stunned, rush close and shoot the shotgun into the glowing orange weak point until it recovers. Repeat this process until it is defeated.