Maelstrom is good, as it gives you a lot of gems, including 5s a turn.
Bottles of water are really good. if you are defending against a lot of earth attacks and assassination spells, this helps a ton.
Rime hauberk is really good. At times i use it on seraphs when mr is not crucial to prevent the frozen heart attacks, and also to allow the seraph to fight units(typically undead) that have a chill aura).
frost brand-hard to beat for the price
boots of quickness-for me a must for all thugs and sc's that do not have water magic.
Water trolls-not a bad deal for 1 water gem a piece. Though it would be nice to see them get some sort of weapon like their earth counterparts. Maybe KO can hold a contest to think up the best weapon to give them.
Sea Turtles-i like them. Expensive at 15 w gems each. but decent mr and morale. and 5 of them can trample smaller thugs easily.
As far as spells frozen heart is the hallmark. Breath of winter can be a very effective situational buff. And of course quicken self is a standard must have buff.
Wolven winter is great to use against Ctis, and other cold blooded races/units. Especially before a big battle. It makes a difference if those Marshmasters are trying to skelly spam in a cold 3 province.
Wolven winter is also good to cast before a big battle if your troops are cold aura troops and you are in enemy dominion(heat).
I am always careful about killing enemy capital population. Especially if it is a capital i border. Since I know I am going to be owning that capital, in a sense I am killing my own future income.