Gremlins - archers are so-so, but they compensate with their numbers. They can repair golems and war machines, which is not bad for a level 1 unit.
Gargoyles - excellent for blocking enemy archers and protecting your own. They have a lot of health + immunities + good speed.
Golems - excellent warriors that can withstand all attacks. You can send them to the front lines and heal them with gremlins. As the hero develops, you can take special skills for golems, giving +2 to initiative and speed. It's a pity that they are slow, but this can be fixed; slowing effects don't affect them.
Mages - experienced casters who should always be in a large number for any self-respecting mage. They cast good and powerful spells. They can shoot without penalty, dealing damage along the line of fire to everything in their path, so be careful not to hit your own units; position your units correctly.
Djinn - everyone dislikes them, but I think that's unfair. They cast various spells on their own and enemy units. You can divide them for better casting (one or two are enough, for example, 1-22). Let one cast, and the others attack when needed. This will also help avoid a counterattack from a stronger unit (fly up to the enemy and sacrifice one djinn so that they attack him; the others won't be attacked until the next turn) + there are many djinn. They are normal warriors, don't underestimate them.
Rakshasa - a powerful unit used for strong defense and attack. They have low initiative, but this can be fixed. They are a kind of tank.
Titans - a level 5 archer; that says it all. Why go to the front when you can protect your own and shoot with all your might? They hit with full force in melee combat as well. The lightning strike ability is weak. The strongest archers have never hindered anyone.
In addition, the heroes of the academy are excellent casters and can support troops with powerful spells. Don't forget about creating artifacts; their strength increases with the hero's level. You can also visit the artifact merchant and find something useful. Well, there's a lot more to say... in short, play the game yourself and find the "+" and "-" for yourself, and then try to use the "+" so that the "-" is not visible.... ;)