Well, I tried to break it up into some phases on purpose, but not as you described it.
First is as you said, a normal phase of sorts with basic abilities. Within this phase is a set of scripted actions that repeat over time or at certain percentages of health. Retreating into his shell creates a momentary lull in battle, which allows time out of his shell to be much more dangerous than an average monster. Then periodically diving into the water to bring adds, also a lull and a change in pace to the battle which requires a different set of tactics and allows the battle to be a tad longer but not a marathon snore fest. The water attack would simply be to make sure the adds are dealt with swiftly, as adds + water attack may kill the adventurers. Then finally a soft enrage of sorts at the end of the battle that puts the adventurer's survival skills to the test. It would be short, require movement/positioning and awareness of the abilities used. But shortly thereafter, Gargantoise would tire and become essentially defenseless.
This then repeats under normal circumstances, but you then have the ability to lock out skills by incapacitating certain limbs. For example, retreating into his shell becomes pointless once you break open a part of his shell. Then he would no longer be able to dive into the water after crippling his legs. And I don't believe I added this, but certainly incapacitating his head could remove his water ability.
This then brings up choices, which are necessary to having tactics. Which limbs do we target first? Are we better suited to deal with the adds, is the water attack too dangerous, or is the shell retreat going to limit our damage and cause an enrage?
You are right, certain monsters do have similar capabilities, which is why I felt the game is already capable of implementing an encounter like this. If we take a look at Elder Mosshorn & Great Buffalo, for example, we see these exact mechanics in use, yet they have very little impact on the battle itself.
I think when we look at certain classes, particularly melee damage dealers, they indeed limited by their role. Your only purpose is to deal damage, but this in turn requires you stay alive as well. So then we have two possible actions available, survival and damage.it seems fun because it is well written but in game terms, I'll spam dps for the combat, exactly as I normally do.
Your only available option then is to make each of these tactics as interesting as possible.
There's probably a lot of potential ways to do this, all I can do is try to outline why the example I gave does so:
Going ability by ability...
Shell Retreat
First, as the others undoubtedly will, this ability can serve to enhance both tactcs. As all NMs seem to have an enrage timer of sorts, an ability that reduces the damage taken will lead to longer battles. Thus survival may depend on whether or not Gargantoise is capable of using this ability throughout the encounter. The specific tactic in this case would revolve around DPS positioning themselves and using certain abilities in incapacitate his shell. This then serves to enhance the role of DPS, as they then must use positioning and the tactics of incapacitation to win.
Diving for Adds
Gargantoise would initiate this event by first moving towards the water. The first tactic involved as a group would be to fight Gargantoise as far away from the water as possible. The second would be to prevent him from reaching the water. This would fall on the DPS to handle, by crippling their legs they could stop him from doing so. The tactic involved would require a shift from another tactic; from breaking his shell to moving to the rear and trying to cripple his legs. The purpose of this is to cause the DPS to move and reposition themselves to react to certain conditions, as opposed to constantly standing in one point to fulfill one tactic, (example breaking Elder Mosshorn's horns). If the adventurer's succeed they are rewarded by not having to deal with adds, but if they fail the event would continue bringing more tactics into the encounter. Once Gargantoise dives, this then gives a moment of suspense and preparation. As Gargantoise emerges, the tank obviously will take the task of handling him, yet the adds will fall on the rest of the group. While DPS classes would be capable of tanking these adds and killing them, casters can CC (Gravity/Bind/Sleep) and deal with them as they please. In addition ranged classes would be necessary for handling the archer sitting atop Gargantoise. Tactics involved in this phase of the event involve burning down adds quickly in order to shift damage back to Gargantoise, and also giving the casters/dps a reason to use tanking/CC skills they would not normally need.
Water Attack
As I stated above, the water attack is primarily intented to ensure the adds become a priority. It is supposed to apply extra pressure (pun intended) on the adventurers in order to accomplish this. While at the same time, the DPS may also decide this ability is too dangerous, and may choose to target the head instead of other limbs in order to remove this ability.
Soft Enrage
The soft enrage is intended to put survival skills to the test. In this case, Gargantoise would have two new skills. First, the stomp ability would be directed at the tank; but, also require the other players to position themselves carefully, as the stomp creates a shockwave in its wake. Then the spinning attack would require the DPS to move away periodically, as they attempt to bring Gargantoise down from his adrenaline induced rage. After either enough damage has been dealt, or after a brief period of time, adventurers would be rewarded by a weakened Gargantoise. The final moments are purely for immersion, as Gargantoise stumbles about, limping along the beach, the adventurer's have won at this point and all that is necessary is the final blow.
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I suppose this became rather long winded; my point though, I wanted to show how while these mechanics are in place currently, and you are correct to point out that tactics are limited but they can also be expanded by creating choices, they are not currently being used together to create situations where choices are necessary. My hope is that the developer's will begin combining these various battle mechanics: Incapacitation, enrages, skills, and scripting; and breathe more life into the dull battles that we are currently attempting to enjoy.
Edit: I suppose I should add; the overall design of a monster like Gargantoise involves creating a monster, whose abilities are very potent, and if nothing is done to it, it cannot be beaten. Thus it creates a scenario, whereby the adventurers are required to use tactics and make certain choices in how and in the order of weakening him. And in doing so, the strategy is then paramount in determining the difficulty of the battle, and each strategy essentially is a different encounter.