Team wise, Keldeo and Diancie have great synergy offensively. Even though Keldeo can’t debuff the sides, Diamond Storm still does enough damage so that by the time you take out the middle poke, the sides will be very low as well. The third unit should probably be a pure defensive tank/healer, or a hybrid defense unit that can give you the missing +2 atk/sp.atk for Diancie/Keldeo. SS Blue and Leaf are two units that easily come to mind. The reason I use Entei here is because it’s a solid mid tier support – I don’t want to use the top tier supports here because they would overshadow the two units that I actually want to showcase here.
One could say that the only reason this works is because of the stage parameter that increases damage after sync, but I will counter that and say that you can just use a 6EX support and achieve similar results.
Pulling for 1/5 Keldeo is probably worth the gems because of its niche tech ability and passives, but pulling for 1/5 Diancie is not advised. I only pulled because I like Hilda and I like Diancie. Both took me 7 multis which isn’t great, but at least I had 100k gems to begin with. Will probably pull 1/5 Celesteela as well.