In Stone Vigil regular, the healer tends to man cannons between HoTs/DoTs. If there's someone with even less opportunity cost, though, such as a Summoner, then they should do it between their DoTs. As there was only a MCH available among ranged, you were the best candidate. That said, should party health start plummeting for any reason, the MCH should have swapped off to tend to the cannons.

In Stone Vigil regular, with the tortoise boss, all should be on cannons all the time. You can HoT before the tortoise lands, but after that you should be able to finish the fight without a single heal.