Combat Medic is a base damage reduction while reviving teammates and is really only helpful to getting further skills. Quick Fix decreases the deployment time for first aid kits which can come in handy. Uppers (Ace), the main reason for this skill tree, adds a total of 10 extra first aid kits to your inventory. Further, they are auto used on downed allies within 5m.
A huge survivability boost, this skill tree is indispensable. Forced Friendship increases cable tie count, and tying speed on hostages. Joker (Ace) is where your survivability starts. Able to convert non-special enemies to fight for you. Converted allies do more damage and reduce the overall time to convert enemies. Partners in Crime (Ace), while having a converted enemy, it increases your movement speed, increases your health, and your converted enemy takes 99% less damage. Hostage Taker (Ace) is another huge increase to you, while having a hostage, or converted enemy, regen health every 5 seconds passively. This skill tree is amazing for group or solo games alike.
Stable Shot is a base increase to weapon stability.