Is it a straightforward tactic? Maybe I wasn't paying close enough attention, but I don't remember it. I should play Heroes 3 again, I've forgotten everything.
The Titans go first and wait, the monsters are forced to move slowly, the Titans shoot, then it's their turn again, the second shot, and a large chunk of the behemoth is gone, followed by a short melee fight with Titus winning.
P.S.: And what is the strongest unit that isn't an upgrade?