The only cross class ability you really use as a Dragoon for endgame content is Mercy Stroke, the lv 26 Marauder ability. You can also use puglist abilities, but for the most part you shouldn't need them.
I would like to see what kind of endgame content that Mercy Stroke is used for. You barely use it in Boss Fight at all due to 20% Hp requirement and high CD. Against trash mobs you typically forget about it since they are down quickly anyway. The healing is ok but not something that you rely on, because that can not cover for any of the boss attack and it's very specific to get that benefit.
Internal Release on the other hand is very beneficial to stack with Blood For Blood and the 10% dmg bonus and -10% piercing resistance. Featherfoot is occassionally useful because some boss attack can be evaded.