400 versus 500 isn't going to make much difference. Especially since the amount of monsters on D:50 means you can't simply retreat & shoot all the time. Speed OTOH is always a benefit - even if you're just staying on the spot you get 50% more damage output at 150 speed than at 100.
Toughness is certainly low - though non-trivial to increase. Minotaur maze would have been a good option but it's too late now. Equip crown of science and hunt swamp hydras, pay Garth ludicrous amounts or wish for !oTo could be used.
Dx should be raised with moss, as a beastie the potential should be pretty good. Practising with the trident will also train it (Dx increases the chance to catch, not for it to return, I think). I find that stats increase quickly on D:50, more than I'd expect from the levelling alone.