For this achievement, you need to score 15 saves in a single game of Lúcioball Remix. This achievement is a little tricky at the moment, as it seems many people are also going for the achievements, and thus, they are only playing the goal. From minute one, you should be playing defense and trying to get saves on any shots coming your way to your goal. It should also be noted that this game mode can, and will be pretty chaotic. At any time in the arena, there can be as many as four (4!!) balls active, which can all be hard to keep track of.
Some tips I can offer to anyone having difficulty with this one:
1.
Don't be afraid to spam your 
ability. Seriously. Your
ability recharges incredibly fast in this mode, and is something you can spam nearly every second, so do not hesitate to use it when the the time is right.
2.
Wait for any shots to enter the green highlighted area in front of your goal. I can't 100% confirm this, but it seems that the game counts a save as "any ball that lands in the area directly in front of your goal." Any ball that seemed close to the goal, but wasn't actually in the area specified didn't seem to count as a save to the game, so I would try to let the balls come inside the highlighted area and then use my
to knock them away.
3.
Don't jump while inside the green area in front of your goal. Jumping in the green highlighted area in front of your goal will make you fly really high into the air. This only serves to get you AWAY from your goal, and, in turn, not saving as many shots. If you are going to jump for whatever reason with your
, you will want to back up into the goal and then jump up, so you don't gain massive air from the pads.
If you're curious as to how many saves you have in a game, just hold
and you will be able to see towards the bottom of the screen how many saves the game has counted for you.
EDIT: Changed button maps to correct defaults, as I had a custom button scheme for Lucio when writing this solution initially.