I see most of people are posting strategy for very fast characters. I got stress on this one (oh you have to beat arcade as fast as possible!) and yes I had listen to others, try Raphael etc. Good tactics!
But today I had play on VERY HARD with Inferno. I did not practice, look on the move set or whatever. What I did notice is: his movesets are crazy!
Everything makes a lot of damage, easy to execute, and fast. Playing for the first time I got like 6'41' and for the the 2nd time I got like 7'53".
Here are some tips for gamers that are rather so so (or not good at fighting games).
Step #1: Unlock Inferno character (clear every story mode for charactes)
Step#2: Learn just few moves with this guy.
I show them in the begining of the video:
Notice that some characters will put up a fight, but no need to worry. Just try again to hit them with whatever move (forward Y,Y,Y,Y, or simple
Give it a try on the VERY HARD, so you can get 2 achievements in 1 play, because it is possible! (^_^)
Again - I did not train for it, no tactics, strategy, just simple "play" and using few simple moves.
Good luck with the achievements!
By the way, on VERY HARD you have to do under 12 minutes to get GOLD.