The thing to remember is that Tritons are pretty cowardly and they will surround the group closest to them.
Put a sacrificial group in front, morale doesn't matter since they will not be able to retreat trough the mass of tritons around them. Or you can put some really heavy hitters with a lot of protection up front which will lock down the tritons for the whole battle. Movement is irrelevant since the enemy will come to you in the first turn.
Deploy your lobsters, tramplers and other heavy hitting units in two Groups at each flank, not too close to the "pointmen" or they will break formation too quickly. Mages and priests are placed no closer to the "pointmen" then that they can properly target the tritons with their spells and act as support (it's crusial to bring atleast two priest units since massive pointmen losses may cause instant routing). Later you can also deploy your own triton units (and seaserpents) in far back on hold and attack rear.
It's the classic "Trident" formation that have won me quite a few wins as Atlantis and it's tough to break in sea provinces.
Your objective is to kill and injure as many tritons as possible, which lobsters do with gusto, and cause them to rout before they can inflict significant losses. It's hard to go easy on the gold as Atlantis so you need to protect your investments well. I modded in an amphiban nature pretender to have Gift of Health available for this reason alone (micromanagement was getting too annoying) but putting a priority to train and equip a druid or the likes lets you use that excellent water9 blessing instead.