What is the lowest Res you'd accept to run a Ploy Skill?

I'd say mid-high 30s to be safe, but low 30s should suffice as well, high 20s and below and I really don't think Ploy skills are optimal

Ill be honest 33

30+ res is what I'd recommend.
The higher Res, the more consistant it will activate.

I have a Kagero who I run Defense ploy on so I'd say high 20's. She can already one shot any mage with resistance higher than her so I use it mainly to one shot bulky infantry units. But I guess the threshold should increase depending on whether you merge your units or not.

Defense Ploy has the lowest threshold since typically units excel in one defensive stat. So for Defense Ploy low 30's is usually fine.

For all other Ploys high 30s would be recommended to have any sort of semi-reliability. Barring odd cases such as that Brave Axe Flier in the Lunatic C&A BHB with his 39 res...

There's a very important question in who you want to hit with the ploy. If you only want to target melee units, 30 res will often get the job done, maybe even 25 because most of them have pretty low res. If you're using res ploy and intending to hit mages, I recommend 39+ because without that much you'll miss against the targets you most need it to hit.

My favourite example is the Kaze build I submitted to his character page. A good number of people would criticize the build for having too little res to land the ploy consistently. I think they are almost but not quite correct. The decision to use spd ploy depends on this assumption:
- if the mage is super fast, such as Nino, Celica, Tharja, Ishtar, and Linde (among others) 34 res will suffice unless they are at +10 merge and using fury, but then Kaze can merge too, especially after one gives him brazen atk/spd
- if the mage has 34+ res, it's probably Deirdre, Julia, Micaiah, Sanaki, some slower mage, and Kaze doesn't need the ploy to double them.

If you understand how to use conditions like these and plan ahead according to your opponents, you can actually increase your efficiency in use of stat points because you don't take more res than you need, using those points instead for atk, spd, def.