The problem with just jamming the water trinity into a solo setup here is that it needs some extra sustain; that core doesn't have enough healing on its own to hold up, and without access to garrison to pick up that slack, the burden falls on MC to provide either mitigation or a lot of healing. Doctor's been done plenty of times, but ML30 Sage hasn't, and being able to mainhand the stamina Opus is an extra bit of healing.
There are a bunch of knock-on effects from slotting Shalem in the build. The biggest one is that Poseidon is pretty much mandatory, since she'll just about never double ougi without his permanent debuff. Dropping Quatre forces MC to bring gravity, which is fine since there's not much competition in MC's subskill slots. Shalem covering stackable debuffs makes Maria in the backline redundant, opening a slot for some other passive.
Sage's mandatory subskill is very poor. Being able to bring double dispel wouldn't have been a commodity even before Beelzebub existing, since Vajra and Shalem could have handled it if needed. Being able to run another healing skill would have been ideal, but an awful shield is the best the class can do. Splitting is a bit more of an HP burden than I'd have liked, but a bit of extra damage on MC felt like it was called for. Blind for both mitigation and Shalem probably would have been better, though.
Opus being the mainhand frees up a slot in the grid, enough to try running the Colomba package. It was alright, but the team probably could have used some more meter to leverage it.
The long and short of it is that no, Sage isn't capable of power healing through. Panacea is a really big button now, but it doesn't scale with max HP like Doctor's green pots do and it's just not enough throughput to cover incoming autoattack damage, especially after 50%. Slotting another healing skill like Just the Cure probably wouldn't have fixed it alone either. There was enough to get through to the finish line, but it certainly wasn't comfortable. The 50% entry turn is probably a good representation of what it was up against.
I think the biggest tell of how many holes the setup has was how lacking I felt the summon grid was while playing. There are plenty of solo teams with effectively only two summon slots that didn't feel short on utility, but I found myself wishing for a Lucifer call more than once here despite already having two healing summons slotted. This doesn't account for how a Qilin would have been nice to have (that first gravity missing was a real bother), as well as how Shalem probably wanted Halmal call for reliability on her double ougi.
Refreshing would have helped disproportionately more in part because Poseidon being able to stack Thunderstruck instead of having it expire all the time. Not much to be done about it, though; seven turns of ougi animations in 180 seconds is a lot to ask.
The damage part wasn't lacking, at least. Both the 31%-25% and 16%-10% skips went perfectly.
water ship 9%/water reactor 10%
0:00 100%
6:39 75%
11:53 50%
17:13 30%