X1 Kai in my opinion is not bad, but largely depends on the player. It is a dive in dive out mobile suit for me. Dive in, kill one, get out afterwards. The non-melee dash W1 can be chained twice if done properly, or roughly 5-6 times if only the first hit connects. X1 Kai is best used as a backup attacker to other melee units. Heat whip is particularly useful for that although screw whips don't do much. Mode 2 is where the main damage is at. It is already fast enough but Ambush parts are still recommended although I go full melee parts (Aspd) with it.
About Skullheart, I have no idea.
爆ぜろリアル！ 弾けろシナプス！ BANISHMENT THIS WORLD!
00 Raiser, 00 7S/G, V2AB, Infinite Justice, HWS v, Master Fuunsaiki, God Fuunsaiki, SR Sazabi, Crossbone X1 Kai, X2 Kai, Harute (Final Mission Type) Stargazer, Crossbone X3, Sazabi
Upcoming projects: Arios Ascalon (100% Complete!), Crossbone X1 Skullheart (100% Complete!)
Playing Phantasy Star Online 2
IGN - ホモゲーバスター