Well, i've just won an EA MP game with Agartha, so i'll share my thoughts.
Scales i used were turmoil 3, sloth 3, heat 3, luck 3, magic 3. You might want to use less extreme ones.
(but stick to sloth 3 and magic 3).
Pretender was awake dom 10 4E6D Risen Oracle (a good chassis choice even if you take lower dom strength).
At the beginning, expand using your pretender (forge him any armor you can), for safety you can do that in provinces with your dominion (he's immortal), independentent archers and troglodytes (groups of 6+). Do not buy any of your other national troops until mid game (25+ provinces usually). If going against astral nations, skip troglodytes too (MR lowered a lot in recent patches).
Buy an earth reader each round and research conjuration until you get to rhuax pact at least (or all the way to umbrals), then thau 2 for site searching spells, then go for blade wind and earthquake. Summon magma children/earth elementals/umbrals as neeed and your gem income allows (umbrals are always needed

.
When you get a little more gold income, get a 2nd castle and buy earth readers there, then switch to buying an oracle at capital each turn.
Recruit wet ones/pale ones/stone hurlers only if you need to go underwater or siege castles. Your main research goal is army of gold/lead, use it on groups of massed umbrals and magma children.
Your heat3 dominion will hurt all other nations a lot more than it hurts you - except for Abyssia, so beware of them.
Be on lookout for air/nature/astral indies and mercs. You will need those to diversify your magic.
Later on, you can try using Ancient Lords as army supporting thugs (with cheap equipment), but only do that if you can't get either Gift of Healts or Chalice - because if you can, Tartarians will clearly be superior.
Try to secure Earth and Fire elementaly royalty, your head start in conjuration and sacred low upkeep researchers should help. If you go for const 6 relatively early, use excess fire gems to forge Lightless Lanterns. If occassion presents itself to cast Forge of the Ancients, go for it and forge like mad for the time you manage to keep it up.
Pitfalls to avoid:
- do not go for a bless strategy. Minor earth bless is nice to give all your mages some reinvigoration, but don't depend on your sacred troops to be useful offensively even with fire bless.
- don't use all your oracles as battle mage (exception is Fire ones, which can and should be casting Magma Eruption). Each army needs just one or two (one casts divine blessing first turn, 2nd army of gold/lead). Use 2E earth readers for blade winding (with boots of earth and or summon earthpower). Better to have 4 2E earth readers than one oracle in the army. Fire readers can cast magma bolts. Water readers can cast frozen heart. Death readers should be busy casting dark knowledge and summon umbral, not being in armies