The Scoreboard in the end of the match has a column for Medals and if you hover on each one you can see what it is for.
Medal: Double Sword: Mighty Vanquisher
Description: Achieved the most KOs in the match.
How to Achieve: Get the highest number of kills in your team.
Medal: Number 3: Triple!
Description: Achieved a 3-play KO streak withing a short time.
How to Achieve: Kill three enemy players back to back without dying in a short time.
Medal: Fire: Relentless Offense
Description: Dealt the most damage in the match.
How to Achieve: Deal higher damage than anyone in both teams.
Medal: Shield: Relentless Defense
Description: Blocked the most damage in the match.
How to Achieve: Block higher damage than anyone in both teams.
Medal: Plus: Gifted Healer
Description: Provided the most healing in the match.
How to Achieve: Heal higher than anyone in both teams.
Medal: Hand: Trusty Sidekick
Description: Achieved the most assists in the match.
How to Achieve: Having the most heals/buffs on players who are kill an enemy. (might not be a accurate description, we are testing this)