You know, if I was in your situation I'd just terminate the challenge and declare a permanent grudge against Ace Trainers. Besides, I'd love to see your new challenge with Black 2 anyway.
The reason why I'm saying you should end the challenge now is that you don't have many options who can deal with both Volkner and the Elite Four, and with Flint being the one you're really grim about (because of his sun strategy) plus the likeliness you'll suffer a massacre against him, it's just best you terminate the challenge at this stage because of your lack of suitable options for new Pokemon, like what happened to you in X where after your Blastoise died you didn't have anything good for Wikstrom and Malva and because of a fear of them delivering a massacre you made the right call by ending it there.
You could take a long break from playing Sinnoh games in general, if you're finding Platinum too hard and maybe after time off you could do a fresh start and attempt the challenge again, like what you did with X.