If it were any easier to get sea troops on land many (myself included) would be clamouring for nerfing the sea nations, especially Oceania.
I have only played MA Oceania, and only in SP, but I believe the following strategy works in other ages and in MP with minor modifications (In MP you need to adept to what your neighbours are doing):
Step 1: Dominate the seas. Unless the map is cradle or full of sea squares there are likely to be at most one other sea nation. I believe Oceania has an edge vs either of the other two sea nations. So dominating the seas should not be too challenging.
Step 2: Cast Maelstrom. You should be ahead in both gems and gold even before the spell as the land nations busy themself fighting each other.
Step 3: cast Sea King's Court. With Maelstrom and the Troll Kings' gem production it takes relatively little time to gather a large Sea Troll army.
Step 4: Land. In MA the sacred troops are quite tough on land too.
I have left out a few details in case some of you want to have the fun of discovering them yourself.
