Good trick against Hagni - use Royal Griffins, and summon Heavenly Guardians ASAP. Hagni LOVES to cast Hypnotize on them, if your army has no other level 1-3 unit with low leadership... which of course, dissipates. She will usually cast Hypnotize every 2. round, saving you a lot of grief (or should I say, Griff...)
All Lizardmen army is great too. Get to Reha as soon as possible - you can sometimes find some of them on Shettara too. Pick the Lizardry Diploma (+5 Attack, +10% crit for Tirex) up on Elon at the Academy, and dig up the Singing Dagger at Reha (you can do it without getting the quest - +5 Attack, +15% damage to all Lizardmen). These two are always available in the game. Lizardmen, almost all of them have the ability "rugged scales" meaning they are resistant to ranged physical attacks, poison, have a small overall physical resistance and a small weakness to fire. Boost their physical resistance with items and the medal Guardian Angel.
Gorguls will be your main damage dealers - cast haste to get them into the fray fast, and target weak units and try to damage units standing behind them. If you kill them, you 50% likely get another chance to act.
Use Tirexes for meat shield, and first round, use up their Roar to stun the enemies - if they have more leadership then you, try to weaken them first with spells. Tirexes eat corpses (except undead) so they are excellent to stop enemies from resurrecting/spawning things from these, also they heal this way. Try to park them near tough enemies to lower their defense. They also increase troop morale, which is also +1 if all your troops are Lizardmen.
Gorguanas work best in tandem with Gorguls. Use Mark of Blood on tough units and then watch them drop like flies. Best effect is using Ball Lightning on an unit with Mark of Blood. Ktahu's Whisper is also excellent when your Gorguls are surrounded, just watch out, it damages your own troops too. Otherwise, they also provide ranged support - pick off weak enemies you can kill with one shot to 50% likely get another round.
Hayterrants are your shock troops and fodder creators. Always attack different enemies who are far away from your pteranodons, to ensure a critical hit. Eggs are tough themselves, unlike the eggs you get from using the Pet Dragon's skill - you can use them to block archers or attract enemies. And, he best thing - the new Hayterrants hatched can also lay new eggs... and so on, and so on.
I used Brontors for 5th troop - they are versatile units, depending on the enemy you face, you can use them as charging battering rams or support archers. You should get Tactics to use them to their best, and they are not that great in arenas with lot of cover and debris, or some suppression battles - however, they can take out a Gremlin Tower with ease when charging from the other end of the Arena. Haste or Adrenaline really improves them. They are also great to cast Target on, as they are very tough, and always retaliate with their special ability.
I found Chosa not that usefull - they cannot move, and enemies rarely attack them, so they can only do anything usefull every 2. round. The Gobots spawned are good fodder, but usually not in large enough numbers to matter.