Aliens need swimming creatures that are immune to any acid water or something. Aliens need more creatures that dont have six legs because so far they all have six legs.There should be a spider or something that can make thread that only helmet equipped humans can see and when they walk into the web they get slow .
Add a centipede! It should start with a few segments. The first few segments share health and are its main body. It can consume humans to gain more segments, increasing its speed and giving it more potent venom. However, if a secondary segment is destroyed, all segments after it are destroyed as well.
That would probably work best with a proper physics engine (like with Newton, or Bullet, or ODE). It might be doable with the current “physics”, I’m not sure how messy it would get with the axis aligned bboxes.