As this is one of the potentially best set bonuses you can get (since it's obtainable with Accessories) I've run into a few scenarios where it isn't proc'ing and have some questions about when it's eligible to proc.

First is Shamael. I put him in 4 piece merciless on Hydra and his A1 counterattacks do not proc it. They're not "true" counterattacks and since they count as Active Skills (for Crohnam) I'm kinda surprised by this. Does anyone know if it's all counterattacks that it can't proc on?

Second is Brogni, because his Shield Reflect is very strange how it works. It can place debuffs from Sets like Cursed so I'm wondering if it can also trigger Merciless? It would get pretty crazy if it could but I don't have the accessories yet to make it work on my account.

Third is Tartura for a similar reason. I'm wanting to bring him to Hydra at some point and his passive reflect might proc it yes/no?

One thing I can tell you and hopefully this helps some is that Mikage can proc it on her Team Up ability. So if she A3s, her first attack can trigger it and I believe I see it trigger on Eostrid when she hits as part of it too. I think it's safe to assume it will trigger on all Team Up abilities (can't confirm on her A1 tho, since I don't have a second SK with the set)

If anyone else has encountered some interesting scenarios, it would help everyone if we can share what we've found please