Jump Guardian is a Sub-Guardian found in Judgment Pit in Dark Agon Wastes. It guards the Space Jump Boots.

General Information

Prior to the Fight

The Jump Guardian's model actually exists in Judgment Pit before you have started the fight. So, you can actually save time by damaging him prior to the fight (you can't kill him though).
The fastest way to do this is by accessing the fight from the intended exit point (Portal Access) by unmorphing at the peak of a Double Bomb Jump in Portal Site.
After you have entered Judgment Pit from Portal Access, jump to the first ledge on the left and position yourself behind and slightly to the left of the light beacon so that your shots will repeatedly reactivate the beacon.
Finally, adjust your crosshair so that it's in between the statue and the right Bladepod.

Once ready, start by firing a fully charged Power Beam shot followed by a Missile.
Do this combination 9 times.
After the ninth missile, shoot four uncharged Power Beam shots followed by a Missile instead. Do this until you run out of missiles.
Finally, walk into the Safe Zone at the bottom of the room to activate the fight. Then, simply shoot the Jump Guardian with an uncharged Power Beam shot to immediately finish the fight.
This section assumes you enter Judgment Pit with 20 Missiles (which you should if you have a full count). If you have less, you will have to improvise and incorporate more charged Power Beam shots.

Head to the Safe Zone right next to Jump Guardian with a DBJ or Space Jump Boots.
Fire 28-30 uncharged Light Beam shots towards the invisible Jump Guardian.
If you have Missiles, you can alternate Light Beam and Missiles for optimal damage. This method also requires less ammo.