I just did the ending at the start, that one was easier than I thought it would be, but then again Crono looks like Goku and he had over 9000! hp.
The ending with just Lucca and Robo was insanely hard but totally worth it. Probably the best ending I've ever seen actually. I saw a brief glimmer of hope when Alya appeared next to them before the fight, but alas she was just standing there watching, and wasn't in the actual battle. Robo only had 4600 HP at max level so I had to put him in the back row. I almost lost several times the final boss used meteor followed immediately by blackhole, lucky me it missed every time (he did manage to kill me once though). Robo using shock actually made it possible to damage the top half somehow, and if I would've had Crono and company with me I would've seen how much HP you assigned to it haha.
One more to go!