Inferno is a "Single Target" unit, his main role is to wipe tanky units.
Passive: Warming Up
Damage increase overtime while attacking same unit
Active: Pew-Pew
All defense units gain attack speed boost for a short period
Tactics: passive - this is perfect on units that require lot of damage - for example high armor / high hp or units that keep getting "heals", the longer it focus the same unit the higher the damage it does.
"Warming Up" resets on unit switch so take that in your considurations.
active - "Pew-Pew" making all other turrets more powerfull since it reduces their reload time, make sure the enemy is under heavy fire to maximize the effect.