Slicer is a "Single target" unit, his main role is to reduce enemies health.
Passive: Slice & Dice
every bullet slices a portion of the target’s remaining health
Active: Deep Scratch
Slices % of hp from every attack unit on the field
Tactics:
passive - it's not designed to kill the enemy units but to lower their health fast so i recommand putting it on 1st line of defense, it goes really well with units such as "Executioner" that can give the finish blow.
active - "Deep Scratch" is extreamly powerfull as it can help reducing enemy health allowing AOE turrets / Executioner to give the finish blow.
I recommend using it as soon as there are many attack units deployed on the field but not to late as it does damage based on "current hp". it goes perfectly with "Frost" skill that make units cluster next to the portal