Actually the rock strategy really does work, I've beaten Vanitas Remnant many times with that strategy. Just go behind that big rock and also, Vanitas Remnant wasn't developed with the smartest AI and thank goodness it wasn't or we would all be screwed XD. The commands you would want to equip at least two curagas and I do believe the rest should be several forms of strike raid (i.e. Strike Raid, Wind Raid, etc Ice Raid.... you get the point, many Strike Raids). The Strike Raids go through that rock and hit the Remnant. Good thing is that they recharge pretty fast, especially if you have the ability where commands recharge faster and multiply that ability. Also in order for it to pass through the rock, you have to be locked onto the Remnant. Keep hitting him and that will keep him in his place, he won't move from the rock. It's a cheap tactic, yes I know however, it gets the job done and in the end, that's all that really matters
Also I won the battle with Aqua so her normal cartwheel is what I most recommend for both secret bosses since it has more invincibility frames than her fire cartwheel.