From reading various posts and playing my own games, I'm getting the strong impression that playing at the highest difficulty levels means knowing about the killer unit combos and skill synergies. I've found what I think is one such combo, and it makes me pretty darned curious about what else might be out there.
If you'd like to share your own tricks (pretty please), I'll try to keep an eye on this thread and edit new options into this top post for easy reading. And it goes without saying that if you prefer to find all this stuff on your own, then don't read futher. Me, I'm happy to take advantage of others' expertise.
Octopus Overlords and the Lanun
I had trouble with the Momus scenario until I found this one: A stack of Stygian Guards and Cultists can easily conquer almost any coastal city. Simply walk them across the ocean to your target and spam Tsunami. While Stygian Guards aren't top-notch attackers, multiple Tsunami's will sufficiently weaken most any defender to the point that Guards can stomp all over them.
Be sure to keep the stack on water so that it can't be counter-attacked by your target's land units. That safety means that your cultists need never fear Assassins or Marksmen; just keep enough Guards in the stack to fend off the occasional suicidal ship.
This is an especially potent option for the Lanun, since you generally only want coastal cities.
Calabim+Order - cabbagemeister
Bless compensates for the Vampire's lack of base strength; Social Order allows cities to be grown ridiculously for Feasting; and Vampiric Paladins are some of the best units in the game.
Govannon+Earth mana - cabbagemeister
Giving Govannon Earth I allows you to spam Priests and Firebows which can be given Stoneskin, the most powerful spell buff in the game.
Svartlfar+CoE (as a secondary religion) - cabbagemeister
You should keep FoL as your primary religion for the GoN civic, but spreading CoE everywhere gives your recon units the Mask ability, which lets them switch Hidden Nationality on and off at will. This means you can attack your buddy with HN, then declare nationality when you're wounded, heal in his cities, and then re-mask and attack again. With normal rangers or assassins, this is unfair. With Alazkan, it's just silly....
Malakim+AV, Empy, and Order - cabbagemeister
An interesting thing about the T3 and T4 religious units is that, except for the high priests, they don't abandon you when you switch religions, even if you switch alignments. This means that a spiritual leader like Varn can switch to AV and build Ritualists, then switch to Empy and build Vicars, then switch to Order and build Confessors. Wait, it gets better. Go research Malevolent Designs, Righteousness, and Commune with Nature. While you do this, your priests will all be leveling like crazy because you're Spiritual. Switch to AV again and upgrade your best Ritualists to Eidolons. Switch to Empy and upgrade your best Vicars to Druids (who can cast Crown of Brilliance!). Switch to Order and upgrade your best Confessors to Paladins. Congratulations, you have WAY too many highly-leveled national units.
Calabim + Empyrean-through-Order - Luckmann
Allows for easily built Radiant Guards that can be upgraded to Vampires. This allows for Vampires with not just Body/Death I-II, but also Sun II. Going through Order will set alignment to Good, allowing for Paladins upgraded straight from Vampires (Sun II Vampire Paladins). This will also unlock Sun III for Vampire Lords upgraded from Radiant Guard-Vampires. This is all above and beyond the obvious benefit of VampireJesus Chalid.
If you'd like to share your own tricks (pretty please), I'll try to keep an eye on this thread and edit new options into this top post for easy reading. And it goes without saying that if you prefer to find all this stuff on your own, then don't read futher. Me, I'm happy to take advantage of others' expertise.

Octopus Overlords and the Lanun
I had trouble with the Momus scenario until I found this one: A stack of Stygian Guards and Cultists can easily conquer almost any coastal city. Simply walk them across the ocean to your target and spam Tsunami. While Stygian Guards aren't top-notch attackers, multiple Tsunami's will sufficiently weaken most any defender to the point that Guards can stomp all over them.
Be sure to keep the stack on water so that it can't be counter-attacked by your target's land units. That safety means that your cultists need never fear Assassins or Marksmen; just keep enough Guards in the stack to fend off the occasional suicidal ship.
This is an especially potent option for the Lanun, since you generally only want coastal cities.
Calabim+Order - cabbagemeister
Bless compensates for the Vampire's lack of base strength; Social Order allows cities to be grown ridiculously for Feasting; and Vampiric Paladins are some of the best units in the game.
Govannon+Earth mana - cabbagemeister
Giving Govannon Earth I allows you to spam Priests and Firebows which can be given Stoneskin, the most powerful spell buff in the game.
Svartlfar+CoE (as a secondary religion) - cabbagemeister
You should keep FoL as your primary religion for the GoN civic, but spreading CoE everywhere gives your recon units the Mask ability, which lets them switch Hidden Nationality on and off at will. This means you can attack your buddy with HN, then declare nationality when you're wounded, heal in his cities, and then re-mask and attack again. With normal rangers or assassins, this is unfair. With Alazkan, it's just silly....
Malakim+AV, Empy, and Order - cabbagemeister
An interesting thing about the T3 and T4 religious units is that, except for the high priests, they don't abandon you when you switch religions, even if you switch alignments. This means that a spiritual leader like Varn can switch to AV and build Ritualists, then switch to Empy and build Vicars, then switch to Order and build Confessors. Wait, it gets better. Go research Malevolent Designs, Righteousness, and Commune with Nature. While you do this, your priests will all be leveling like crazy because you're Spiritual. Switch to AV again and upgrade your best Ritualists to Eidolons. Switch to Empy and upgrade your best Vicars to Druids (who can cast Crown of Brilliance!). Switch to Order and upgrade your best Confessors to Paladins. Congratulations, you have WAY too many highly-leveled national units.
Calabim + Empyrean-through-Order - Luckmann
Allows for easily built Radiant Guards that can be upgraded to Vampires. This allows for Vampires with not just Body/Death I-II, but also Sun II. Going through Order will set alignment to Good, allowing for Paladins upgraded straight from Vampires (Sun II Vampire Paladins). This will also unlock Sun III for Vampire Lords upgraded from Radiant Guard-Vampires. This is all above and beyond the obvious benefit of Vampire