If you want to play Phantom, go ahead. I don't think it's wise because I think you're new here, but I want to help you out as much as possible because I don't think others would be as supportive. Before my proper tips, I'd tell you to familiarise yourself with the maps, mechanics and nuances of this game by playing other, more effective mercs like Saw or Bush.

(A lot of people may not agree with the tips I'll give you, but I speak from experience with what works for me.)

** Your cloak provides little stealth**

Your cloak provides a constant hum that will alert enemies of your presence. Unless your enemies are distracted, it's preferable to stay uncloaked.

Your cloak's visibility is directly proportional to your speed. People can see you run, so no matter how counter-intuitive it may sound, don't use your cloak to traverse open spaces because you are either a visible target or a slow one.

You have a gun

One of the reasons why many new players are attracted to him is because they picture him as a melee class. This will get you killed more often than not. Your gun is your primary means of attacking, strike from unexpected places and they have little time to react.

Rushing in to melee someone is ill-advised because they can hear you while cloaked and will, more often than not, gun you down. The only times I would say to use your melee is when you successfully catch people of guard and manage to hit them once. After that, you finish them off with your primary or another swipe.

Your cloak doubles as armor and as an EMP

I typically activate my cloak right before I start a fight. Doing it earlier will alert them of your presence. I typically do this to check if there's anything important (like stations, mines or turrets). Using that data, I make the decision to attack or to retreat.

I don't really know what other advice I can give you because the way I can make a lot of snap decisions regarding whether or not a course of action is beneficial to my team. This comes from a lot of practice with the other mercenaries and familiarising myself with certain situations that can pop up in a game and knowing how to react to these situations.