It's not hard, it's just time consuming as all hell... and boring. I've done He Fei a couple hundred times now, and as you say, running out of characters you like playing makes it even harder. There are a lot of CRAP characters to play in DW6 (Wei Yan, anyone?).
Chaos mode is pretty easy with SSX or Yue Ying and a 90+ Power Flash weapon.
Quid quid Latine dictum sit, altum videtur.
Killing with a vehicle mounted gun is nothing, doing it while airborne is impressive, but upside down while airborne? F***, THAT's an achievement!