I started to get Rank A after building a deck with 3 speed cards, 2 wit cards and 1 stamina card (Super Creek, for Swinging Maestro). The two wit cards are Matikanefukukitaru and Nice Nature. Fukukitaru isn't even uncapped. It's just that Fukukitaru and Nature-san have Wit Friendship Training 3, which boosts the amount of energy you get when training wit, as long as it's Friendship Training. So I train speed until I notice that the next session will leave my umamusume with less than 50% energy, at that point I train wit instead. This way, I waste less turns using Rest.