[Russian_Comrade]

1 Sea Troll costs you 1 W gem and ~8 RP. Summoning 100 Sea Trolls = 100 W gems and ~800 RP. It is a very heavy investment - mostly because of casting time. Also, large unit and attack skill 9 - so they don't do lots of damage. I for one found only two uses of those guys - serving as a lightning rods (a small number of those guys standing in your forces to soak bad magic and arrows) and expanding underwater - and that army have no water in sight. And they use gold for upkeep, so it is damaging for your economics as well.

I'd say that summoning 60-70 Winter Wolves is much easier (6 or 7 turns of casting time) and have more capability to create problems for your opponent.