is donphan worth powering up now?

He's a bulky defender with a decent attack and without a double weakness. I don't think he's specific enough to go raiding with. Mine's powered up for the CP, but that was back when it meant something. I wouldn't do it now, even though he's cool.

Meta is currently about getting strong attackers for raids. I don't see much of a point in using TMs and stardust on other Pokemon than Machamp.

I powered up mine to lev 33 and I use him as 4th attacker into Tyranitar (after 3 Machamp)

Sometimes I put Donphan in as #6 against Snorlax in raids. He sees no action. CP gets up there though so could be worth it if you don't have good ground Rhydons.