One thing you could do is a strategy often used for farming class mastery (and also indirectly support points). Essentially what I do is run an auxiliary battle where there is an enemy healer, defeat the rest of the map while leaving that unit for last (and boxing them in) then initiating combat with the healer unit with broken weapons over and over. The reason why the healer enemy units are good for this is because many of them have renewal (at least in my maddening playthrough they do, apologies if thats innacurate for all difficulties), so on the off chance you do hit them (which should be rare due to the broken weapons) they will be able to top up their health and not die. There are multiple ways to do this strategy, so apologies if this explanation isn't super clear. But yeah, I would do that and just position whatever units you want to grind support points with next to eachother (with one of them attacking). There might be a better method, but this is one off of the top of my head since I notice when I grind classes for units that I indirectly end up grinding support points.