How do you kill these type of enemies?

If they are the ones I am thinking of, there is no way to kill them, you can just safely bounce off its head, and work around them.

Apparently you can kill those ones.

Hm, then maybe they are the ones that can be killed if you have the portals that make you larger?

You need the time portal gun and the time suit. Have multiple versions of yourself jump on it at the same time. Being big lets you damage big enemies but not do more damage to small ones.

edit: Just tested this -- you also need to jump on it when it's not currently recharging. It heals so fast no amount of concurrent jumping will kill it. So just jump on it when it's not on the charger.