Forgive me for just copy-pasting a post I made some time ago, but here are my tips for playing the Atronach:
- Your aim is obviously 100% spell absorption, but tbh anything above 70-75% does the job. At the moment my char has 82% spell absorption and is virtually never hit by a spell, much less two in succession.
- Be mindful of your elemental weakness, and look for appropriate enchantments, but be aware that this ceases to be an issue with enough spell absorption.
- Alchemy is vital. If you intend to use Destruction as your primary damage source, use alchemy to obtain buffs like shield, night-eye and detect life, thereby sparing your magicka reserves additional strain. Supplement your healing spells with potions. And above all, keep up a good supply of restore magicka potions. Harvest flax whenever you see it; with restore magicka, feather and shield effects it really is the wonder seed.
- Keep your eyes peeled for ayleid wells and wayshrines. Try to visit as many wayshrines as you can in order to activate the relevant chapel altars, which brings me to...
- Praise the nine! For an atronach, chapel is by far and away the best place to train, particularly once you've visited a few wayshrines. Those blessings are potent spells for you to absorb.
- Watch your infamy: wilderness wayshrines will no longer grant you a blessing once you gain even 1 infamy, this doesn't prevent you from using divine altars in chapel though. However, allowing your infamy to exceed your fame will (I believe) prevent you from being blessed altogether, so you may want to keep that in mind if you were only thinking of doing DB quests, for instance.
- Keep a good supply of Welkynd stones for emergencies. My character never passes up the opportunity to explore an Ayleid ruin for this reason; I have dozens of Welkynd stones stashed in each of my houses and always try to keep ten on my person at any given time for emergencies. (Ten is almost certainly overkill tbh, I've never used more than two or three in any given dungeon, so if you're going down the battlemage route and have issues with encumbrance this number can be reduced as needed.)
- Learn which enemies are your friends: keep enemy spellcasters alive as long as possible; an atronach should be positively happy to encounter any kind of ghost (wraiths in particular have some tremendously powerful spells that can fill your magicka reserves in one shot); the same is true of will-o-the-wisps, scamps, atronachs etc. When fighting these kinds of enemies, you're always aiming to finish the fight with more magicka than you started.
- Make your magicka count: magicka efficiency is of the upmost importantce to an atronach, it is almost always cheaper to cast two weak spells than one moderate one (this also helps speed up skill advancement, which may or may not be a good thing depending on your play style).
- Traps are often a good thing: dark welkynd stones and those flame tower things you find in Obvlivion gates are another easy way to recharge your magicka.
- Do the Aleswell quest as soon as you can: it's a free home with an ayleid well just down the road, perfect for an atronach.
- This one's borderline dark side and I never use it myself as it feels like an exploit, but feel I should mention it for completeness: with a decent mysticism skill you can replenish your magicka by just spamming a telekinesis spell repeatedly. I only noticed this myself when using telekinesis to grab difficult-to-reach welkynd stones, and promptly filed it in the same category as 1-second drain intelligence spells in Morrowind.
For restore magicka potions, combine any of the following ingredients: flax seeds; bog beacon asco caps; fire salts; steel-blue entoloma caps; stinkhorn caps; void salts; water hyacinth nectar.