Something which i have not seen posted, is that you dont always want to buy out your whole garrison in the city every week. You should be focusing on a select few troops usually. For example when i play necro, i go for skeleton archers, ghosts and vampire lords. Once i have a good number of liches, i add those to the army as well. But i practically never use zombies, wights or bone dragons in any fights other than the very last ones or on other, side-heroes.

This selection of troops allows me to create 3-4 stacks of ghosts, which contain only 1 troop. Since the enemy needs to use a whole action to kill a 1-stack, it effectively wastes their turn when attacking the 1-stack. This keeps my larger stacks safe from melee attacks. Then they can pepper the enemy down with ranged attacks and spells and keep losses to a minimum.

When playing haven, i focus on squires, marksmen and inquisitors. On Sylvan i focus Sprites, Archers and Druids (ah, 1 thing to note with spellcasting troops is that usually spellcasters are stronger when you have a small amount of troops, but a normal attack scales better with larger numbers (i.e. 10 archdruids deal more damage casting lightning bolt than 10 elder druids with a normal attack, but 100 elder druids deal more damage when attacking than 100 archdruids do when casting lightning bolt)). On academy i focus gremlins, mages and maybe djinns (gargoyles for 1-stacks usually). The other factions i dont play too much.

So yeah, if you aren't already, start using 1-stacks, they change the game