I've mostly been joining random raids with this support Gardevoir set, and had a few dozen successes with it to max out on tera shards:
Gardevoir @ Covert Cloak
Ability: Trace
Bold Nature, HP/Def
- Will-O-Wisp
- Heal Pulse
- Charm
- Helping Hand
There's one important caveat of timing you should be aware of if you use this set. At the start of battle, when everyone gets sent out and the ability popups happen, you see your own ability popup with a standard duration, but most other ability popups appear and disappear more quickly. So I'll see the popup for Trace, then for Shell Armor, and the text message about tracing the ability, all with their normal duration, and then Samurott will use Focus Energy after it's all done. But everyone else will see the popups go away in just a second or so, and they'll see the Focus Energy animation start, and finish, sooner than it does for me. This means that if one of the teammates is dead-set on rushing to get the first move in as fast as possible, they are sure to be able to do so faster than I can possibly use Will-O-Wisp, and if Samurott's first retaliation against that player (plus the possible Bulldoze if move 1 was a damaging move) ends up immediately KOing them before I can put the burn in place, the timer drain will see to it that the shield comes out immediately, and Will-O-Wisp will then fail. Any time that happens, it doesn't bode well for the success of the raid. Even with other players also using entrance abilities (Koraidon, Gyarados, sometimes Corviknight), the fact that Trace's mechanics go through 2 popups instead of 1 means the shortening of the Trace ability popups has a bigger effect on them than their popups do for me, and I'll still get held back from the move menu later than anyone else, if they're not patient.
All of Samurott's moves are 100% criticals except Bulldoze and Megahorn, unless we have one of the abilities that blocks them (big thanks to Samurott for bringing one of those abilities itself, so it can be traced off!) For everyone else who likely didn't bring those abilities, it's awkward because attack drops and screens won't do any damage reduction whatsoever against criticals. If a supporter is going to provide any form of damage reduction for the others, the ways we have of doing this are very limited, and mainly consist of burn, and blue cheers. Even criticals still have to respect the effects of those, hopefully keeping the team alive longer so the timer doesn't drain quite so fast. Once the Swords Dance is up, Charm can also cancel that out to go back to +0, but any reductions into the negatives are only likely to affect the damage for me.
With the burn in place, Megahorn (the only move Samurott will want to pick) does less than 25%, and a blue cheer knocks that down further, enough that my own health isn't an immediate concern and I can figure out the most effective thing to do with the partners I have. Sometimes WOW misses, of course, but there's a bit of a saving grace here in that if everyone else is going for a setup nuke with moves like Screech (and yes, you'll even see some Belly Drums in these raids because of course you will), missing WOW means there's no burn damage. If there's also no damage from anyone else, that means Bulldoze doesn't come out yet and I get another shot fairly quickly, without having to wait through that long animation with all the sequential stat drop messages.
Once the shield goes up, if there's not a blue cheer up yet, obviously getting that is the first priority. Otherwise it's a case of looking around and seeing who needs Helping Hand or Heal Pulse the most--if the damage is spread out all around, it'll have to be a green cheer instead, but Heal Pulse is preferred where possible due to the lack of randomness, and larger supply. To that end, Covert Cloak as the item accomplishes very little, but preventing the stat drop from Bulldoze has the effect that all Heal Pulses (and after the shield is gone, reapplying burns) outspeed Samurott's moves, just as cheers always do, so if things get really awkward I can at least take a sack and still get off one last move beforehand.
Other niche mechanics I can do...if Ceruledge shows up to a raid, that's probably not ideal, but once I have the light cover in place so it's not being shot down so easily, I'll typically throw a Will-O-Wisp their way, to turn on Flash Fire. This is effectively a permanent Helping Hand for the rest of the battle, giving them more damage (and therefore more recovery) with the Bitter Blade spam which is presumably why they brought that pick in the first place. Hopefully that makes their slot lower maintenance so I get some time to check on the other two. For a while, I used to run Skill Swap instead of Charm, with the idea being that I could donate the traced Shell Armor to an ally who could really use it, then steal the original from the boss itself. But things rarely went smoothly enough that there was time for three moves pre-shield, and being stranded without critical immunity when the shield is up defeats the point of the set. It was also possible to do cheeky stuff like repeatedly swap Intimidate to get stat drops that worked even through the shield, but this was not only reliant on having an Intimidate partner, it also used up several turns where one or the other of us lacked immunity, so the stat drops weren't all that useful anyway (see above).
There are a few other supporters I can see make use of the "steal boss's ability" function.
Stantler is an intriguing one that gets both Role Play and Skill Swap, which seems redundant but there's a niche for running both moves at once. You can Role Play to steal Shell Armor
without giving back an Intimidate that your physical-attacking teammates (other than Defiant Annihilape) might not like, then Skill Swap with a teammate and Role Play an extra copy of the ability back from them, instead of from the boss (which would fail if it's shield time already). In this manner, you can even extend the critical immunity to multiple teammates if you want, though it's impossible to swap with stuff like Koraidon or Iron Hands, and you don't want to overwrite Anger Point until you're sure there are no more poz-wipes coming (and by then your teammate should be in striking range of the finishing blow anyway). Stantler can also use Sand Attack, which isn't as good as Mud-Slap but...hey, that's another way that it's possible to protect your teammates, get them to fish for misses every turn. And oh yeah, Stantler gets to take advantage of Eviolite too, so it shrugs off pretty much everything.
Polteageist lets you swap to take Shell Armor and give back a much Weaker kind of Armor, so the boss can perpetuate its own defense drops thereafter. However, giving them speed boosts at the same time might not be such a good idea, because if your teammates suddenly start getting outsped and KO'd before they can move at all, that's just going to bleed clock. Being even more physically frail than Gardevoir, with an outright weakness to Night Slash, isn't great either, even if you can heal back with Strength Sap and get some stat drops at the same time (which, again, only really make a difference for you, not the teammates). Having to use the first turn to steal the critical immunity, instead of getting it for free with Trace, is such a big cost.