hey guys, sorry I was to come back here and post my method. I have completed the game 200/200.
For Feral Fighter, the easiest way to get it is to simply beat the game once to unlock Roscharch VS Nite Owl.
then simply play with a 2nd controller on, so that you have a dummy Nite Owl to spam all the required combos for the Achievement!
Yes, i have wasted several hours trying to get that achievement too, and out of patience, just when i was about to leave the game, i thought of this and it works perfect.
I can also confirm that you can link the combo, so use the list of linked combo I posted in initial Post, they will help you cut time off. I also suggest starting with Extra and Maximum Damage.
For Martial Artist achievement, I tried the same method but haven't found a correct way of doing it as Nite Owl is way too powerful and kills off Roscharch before doing all 8. I have linked combo for Nite Owl too, i can post them here if someone wanna attempt the method in the Rorscharch VS Nite Owl mode.
XYYY (this is 3 combo finishing with a Knock Down, you need timing for this)
that's all 8. you can try to link more, but i found it hard to time off.
I personally got this achievement in Chapter 3, fighting the big belly guys where I used the Extra + Maximum Damage, and the girls all other combos since they rarely block(they sometimes counter). The Girls are good to pull combos on since they all try to mob you, whereas in chapter 2, some gangsters will hold off a bit further until you kill a few.
If you guys need any more help with achievements in either Part 1 or Part 2, msg me up mentioning this forum on xbox LIVE. Since I completed this game already I won't be checking this part of the forum much.
Good Luck to all, please include this in the Achievement Guide, I'm sure the Feral Fighter I found IS the best method to get it.