Depends on content, party composition, etc... Hournes First Division/Scion/Ramuh/Golem would be good picks generally.

Trousseau could also be good depending on team whether magic or slash is doing more damage

Wouldn’t use golem generally, maybe the 15 def/spirit rare vision card instead for tankability. Or the MR agi slash attack card for soul prominence and turn frequency

Encounter of Heroes bruh

Encounter of heroes (Rain exclusive Vision), that vision 100% exclusively designed for him.
For slash resist you can use "Solidus, hourne first division" Vision on other mage, or you can use it directly on rain.
For magic resist you can use his passive, rain SPR stat already high enough 36-40 (using passive skill spr up) even higher if you use his own TMR ~54-56.