[Lazy_Perfectionist]

As LA Atlantis, you can often easily afford to get more forts up and running, since you've got very poor land forts.

Once your income allows, recruit Angakoks every turn. A lot of functionality is bundled up in them and their random pick. The more you have, the more utility. You may even get lucky and get air 2. Treasure that one, since there aren't any low path air boosters short of empowerment and storm power. Auspex, was it?

First and foremost, sailing. It may seem redundant with amphibious, but it is an awesome ability - though dependent on the map. The sizes are often wrong on a randomly generated map for it to be any use, though. Keep this ability in mind when shaping your empire. Even after you've used sailing to good effect once, the opponent often leaves some vulnerabilities. If you can't use it to raid, or defend a landbound fortress, you can often use it to cut off a path of retreat for an enemy army who has encroached on your territory. You need to focus on making Tungalik's effective, since they're available everywhere, but Angakok's are your game changers.

Random 1' (110%s) magic in air, earth, death, and water is essential to keep in mind. While every Angakok has some effective spells, there are a few key battlefield spells I will point out, though there are fun things you can do elsewhere (such as use up spare earth gems with Blight and Earth Boots).

Evocation Six & Seven opens up the Wailing Winds and Wind of Death. These spells are fairly cheap to cast (avg. about three gems, plus probable booster item), at 100 fatigue, and affecting the battlefield. While Wind of Death affects friendlies, decay isn't really a problem for Atlantis. Even a 25 turn battle will only add 125 years, and even your Angakoks and Mournfuls can take some extra age.

Since an Angakok can easily bring 100 troops or more along with a little experience, you can make some devastating strikes, as long as you remember Wolven Winter, which I generally cast once or twice on the same turn I order the invasion.

One word on Rigor Mortis (fatigue, battlefield enchantment). If you think it might combo well with Grip of Winter: Don't. It'll ravish your mages. On the other hand, you can theoretically twiceborn every mage you own, and once they're wight mages, they get cold chill, 100 cold resistance, and undead, saving you from worrying about partial CR. No more sailing, but they occasionally gain a free pick of death with the associated immunities and weaknesses. If you can afford to invest in an entirely undead, low mobility army, Rigor Mortis may be worth considering, and comboing.

Darkness is, of course, available, but that's always going to be quite expensive, costing 4 to 9 gems depending on whether you need the mage to stay conscious afterward. It's probably best to plan on Darkness dropping your mage off to sleep. Worth keeping in mind is that Darkness is Alteration six. At the same time that you research this, you also get access to, W1 Frozen Heart. Frozen Heart is castable by every Tungalik, and has 100 precision. Even with about a 40% precision penalty, this'll aim true.

Also at alt six, consider casting Iron Bane (E3). I'm not absolutely certain, but I think it doesn't affect most of your armor. I presume the various ice armors are magical, and not affected, though I hope to see soon, in my game, a definitive test-case.

Going up against Fire magic?
With a couple spare gems, every Tungalik can cast Rain. Given the choice between casting it with a Tungalik and an Angakok, I choose the Tungalik every time. Sure, it may cost me an extra gem, but its generally worth it to free up your generally rarer Angakoks for more important, or repeatedly cast spells such as cleansing water.
An Angakok with an earth pick can cast Fire Ward (enchant 4), giving you 100% FR.

If you've got several air Angakok's who don't need to cast anything else (no huge hordes of longbowmen, or you're facing another archer deprived nation such as Ctis), Freezing Mist is a wonderful spell for Evocation 3 and only 20 fatigue. Repeatedly castable without any boosters or gems.

Don't combine Grip of Winter with Mournful, unless you're willing to cast Winter Ward, or Warriors of Neiefelheim. It's a mistake I've made once or twice.

By the way, Mournful will suffer the encumbrance effects of severe cold in a three scale environment. That affect is ~4 additional encumbrance. This, along with interesting independents is one reason not to take cold three- unless its your opponent with those interesting independents.