Well, it's easy in the sense that you can win it by just letting your enemies wear themselves out on the griffons. But if you want to get griffons for later scenarios, you have to fight both the enemies and the griffons *and* keep them away from each other, which is quite a bit more difficult.
The trident is usefull in Bay of Pearls and Ford of Abez, I did'nt use it in during The siege of Elensefar (wasn't necessary at all) but I used the 2 mermens in A choice must be made : they guarded the bridge against undeads and did a marvelous job (one got to Triton, the other to Hoplite)