I will go briefly on units
Useful Units:
1) Element: They are necessary tactical assault units. It is the first unit that I train at my capital so does everybody else. But they have no strategical value in the late game hero stand off.
2) Big Three: Red Dragon-(Pegasi), Devil-(Unicorn), Grand Archon-(Archon). Stack up gold to 2000 early and try to receive one of them with a hero offer. If you get one from the ruins that saves production as well. Try to buildwhat you don't have .
3) Catapult: Every city will be +15 soon.
4) Medusa: 65% Ambush + Fear, fantastic unit
5) CRUSADER: This is the most underrated unit of the map. Much better than any other two turn unit. From now on I will plunder Minotaurs.
6) Wizard: Annoying tactical assault unit. They also helps L2-L3 ruins.
OK Units
Spiders, minotaurs for defense.
Ghosts for non-hero city defend and ruins.
Heavy Infantary-Scouts-Orcs as early units.
Dwarfs for mobility at certain regions.
Sandworms: Good for +15 walls.
Doubtful units to train
Mammoth: Good defender and +10 group warding rocks. Just too expansive
Green Dragon: Good to have one in the hero stack but attacking elements are already 40/3 and I have plenty of them.
Elephants: +2 move and morale is good but most the chock points and towers are on the hills.
Demons: Who has the time and money to put together 5 of them. And most rangers get +10 Anti-Air making them effectively 45/2.
Yeti: Kind of good for snow mobility and +30 warding is ok. But I often get them from L1 ruins and no need to train.
Bad Units
All the other ones
Worst Unit
Gryphon: Very slow to move and produce. Good money to plunder.
Maybe somebody can go over Heroes. Ranger-Assasin are the obvious ones. The game either ends with a Rush or with a Hero stand off (often called Alamo). It rarely ends with a Harassing Hero razing the homeland and crippling the production.