Mortis is not too strong in this game mode, but his mobility can be intimidating - Break boxes and hide early, do not use all 3 dashes too quickly, as you should save some to escape if a strong duelist approaches or you notice you are being sandwiched.
Allow others to fight each other as you amass boxes, Mortis is the fastest brawler so you can quickly go from box to box, but keep in mind he is not the fastest clear.
Try to quickly get the first star and retreat, a single star can sometimes be a game changer, but do not foolishly die just for the star, you will only achieve giving the enemy team two stars and leaving your teammates in a 2v3 until you return.
Go from sides to get a flank position, you may be able to assassinate an isolated target and escape with the stars, but be very careful to not hand over stars.
Wait for your team, a coordinated team will still be able to defeat a Mortis, even if you think you are invincible with your dashes
Force enemies to miss shots on you, this is protecting your teammates and it weakens the offensive potential of the enemy team, allowing for your team to win
Your super move can allow you to save yourself, invaluable if you would have been about to hand over 7 stars