Frost is a "AOE" unit, his main role is to slow the croud and give the other turrets more hitting chance.
Passive: Freezing Breeze
Slow down units
Active: Feeling Cold
Stuns all attack units for a short period Tactics: passive - prolonging the time enemy unit spend on the lane (slow) means your defense turrets can apply more hits. make sure you position the Frost in a place surrounded by other turrets.
* note - there is a "Slow Cap" so make sure you dont "Stack" Frost's next to each other (no need for more than 2 frost next to each other).
active - make sure you use "Feeling Cold" when there are many enemy units under heavy turret fire as you wish to have those turrets keep firing. if you play it too soon there probably wont be enough turrets next to the attacking units.
* note that this spell apply on units that should spawn as well making them "cluster" which can be good with AOE turrets