Here is the full list of all 12 Mortal Blitz: Combat Arena trophies - there are 7 bronze, 4 silver and 1 gold.
1 multi kill with a rampage in [Quick Match]
Arena Pack open for the first time
Place 1st in [Quick Match] mode
Finish tutorial
Reach Lv. 10
Use a starter weapon
Place 1st in [Quick Match] mode 5 times in a row
15 kills with a machine gun in [Quick Match]
10 headshots with a minigun in [Quick Match]
7 chain kills with a shotgun in [Quick Match]
3 revenge kills with a launcher in [Quick Match]
4 double kills with a railgun in [Quick Match]