[Agema]

Okay. For non-hydra units:

Slingers are rubbish. Just buy independent archers if you want missile troops. However, as an air nation you'll probably want to look at casting storm and storm power, which makes any archers a bit of a waste of time.

Principes should be the backbone of your army. They are good quality troops, quite capable of taking on good quality infantry of other human-sized nations, and should smash weaker, basic infantry.

All your three 'basic' (stats of about 10) units are pretty effective, and have javelins for a bit more punch. Velites are your best "chaff" units, but because of their okay armour and huge shield can still hold up reasonably well in melee. Alae legionaries and hastati are better armoured so more durable, but basically the same, and the res cost means if you need a lot of troops quickly, go for velites. When facing better armoured opponents, favour hastati over alae as their short swords do more damage than spears. They have better morale as well, but not by much.

Emerald Guard and Serpent Cataphracts are shock troops. Emerald Guard have magnificent stats, but their slowness (stratmove 1) means they're best suited to defensive actions where you want some great troops to hold up as long as possible and soak up plenty of punishment. If you want to take an army somewhere, you'll want the cataphracts in order to be able to move at a decent speed. On the battlefield, for the extra cost of cataphracts, compared to emerald guard you have similar stats, get two attacks, half the encumbrance, and the lizard carries on fighting after the rider dies. They're not great cavalry in terms of them not being particularly fast, but they're still decent for putting on the flanks with attack rear orders.

Gladiators and battle vestals are a waste of space. Gladiators are powerful but as they disappear after one fight, they're a bad waste of money unless the situation is really urgent. Battle Vestals I guess could fulfill the role of "chaff", except that as sacreds you can't possibly build them fast enough.

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Magic. Pythium is basically an air/water/astral communion race. Only your arch-theurgs can fire good mid-level spells out of the box. Even then, it's only in astral they're really useful unless with communions (They'll generally be A2W1S4, A3W1S3, or A2W2S3.) Astral has useful BF uses like paralyze, but it's not good at wholesale army devastation until late on. Theurgs are also handy BF mages with enough slaves. Bear in mind most mid-level communion spells require A/W/S 3: if you're using theurg acolytes you want 8 for the +3 (otherwise they take additional fatigue from stuff like thunderstrike), or otherwise cast power of the spheres for the extra +1.

Pythium is a very good research race. Your magic users are sacred so have half upkeep costs, and theurgs and theurg acolytes are deliciously cheap. You want Arch-theurgs mostly for battles and casting rituals, not researching. Start your hiring with theurgs to get research ticking quickly.

There's one big problem: theurgs are old, arch-theurgs severely old, so they die, regularly. Later on as the midgame approaches and you have multiple castles, you probably want to think about hiring theurg acolytes (3 research). This is purely because they don't die of old age, they're still cheap, and they're the main slaves for your communions. Make sure you always have theurgs and arch-theurgs around, though, as the acolytes will be not much use without the better mages to cast better spells.