Trooper is a "Soldier" unit, his main role is damage the enemy nexus.
Passive: Double Trouble
This unit comes in pairs
Active: Reinforcement
Summons additional troopers to a random portal
Tactics:
passive - every trooper you deploy will be "doubled" meaning you can create "mass" waves
active - "Reinforcement" is a tricky one as its "Random", the enemy must think twice where to build turrets as "Exposed" lane can be taken by surprise.
Current recommendation is to use this skill as soon as possible unless enemy have "Slicer" than can hurt the units