For all those who out there who don't know, the heat bar on SnS fills up based on the amount of damage taken while guarding. Well, many avid SnS users may know that, but what you may not know is that damage reductions of all sorts reduce the contribution to that gauge. This includes, but is not limited to, defense, resistance, and reduced damage taken while guarding.

So little tip for those trying to increase the damage they do with SnS, do not use the Plume set. A full set pretty much makes it impossible to use your heat bar.

Furthermore, when choosing magis, an all HP ATK build is much better than a build with defense. In many cases, it could also be very beneficial to leave your SS armor at lvl 1 so you can receive the benefits of the skills without sky high defense.

TLDR; It can be beneficial to purposefully lower your defenses in a variety of ways so that you can use your heat skill more often.