Tactical units are used for killing enemy troops, you buy them with rubies in the smugglers den. There is only one way to employ them, send them to your enemy and watch the carnage.
Might be important to note that each one is for a single use only. Whether you make a full kill or a complete miss, you're not getting that unit (nor the rubies you bought it for) back.
Note fireships are 500 rubies but its generally best to get them when they're on sale 50% or better, then stock up so you have them. Speaking of I need to stock up used to have 100 at all times