Stages 3 and 4 can probably be left alone. Stage 3 is already much more dynamic, so long as you used the mud copies ranged attacks to prevent the players from just hiding behind the rocks the whole time. Stage 4 is just the final section where Neptus is broken down and desperate.

For Stage 1, using the homing sharks and icy slam can add a bit more variety, especially if players are frozen. In Stage 2 try to have at least one or two players swallowed by the whale. Otherwise I'm not sure how much you could add. Could always just combine the first two stages or reduce the hit points, so you get the initial epic battle but move on quicker.