All you have to do is make a trial that is Hard as hell. (ha! a pun!)
Does Dante get health drops? No
Mana Drops? No.
How many lives? 1
What difficulty? Infernal
How many bad guys? Max
What strength bad guys? Max
(Just RT to duplicate wave till trial reaches GOLD)
and Kennansoft's solution is almost right.
The acheivement says to make a single player Dante GOLD trial.
So you can basically follow Kennansoft's step-by-step guide except where it says:
"After this Press B and then while on the Wave Navigator Screen press RT numerous times to Duplicate that wave, watch the medal on the side till it goes ---Platinum--- then press 'Back' and select Save & Exit."
You only want to make enough waves as to where the trial goes GOLD not platinum.
Hope this helps! :)
also when you come up with a name for the trial, make it original. Otherwise,
if a trial of the same name is in the server,
you might not be able to upload it.
Thus no acheevo.