You will pick up the Power Glove at the Solar System Observatory - one the antennas you must power up.
You will be trapped in a room and must use the it to defeat the crabs before you move on. After that it will permanently be in your inventory as a melee weapon.
To get a takedown you must target crabs with the red weak area on their back - it will not work on armored crabs or Queens.
Simply jump or boost high enough right above the crab and you will get a prompt for
Takedown. Click it and you will go into a melee animation.
Rinse and repeat 10 times!