Secret Mission 17 made me just straight stop playing this game.  
 
I was trying to get them all on my first play-through, but I must have fought Yeagar like 10 times... and failed the Secret Mission every time. 
 
I'm going to try again today, but do you guys think I should just skip the Secret Mission and finish the damn game already or does anyone have any really good tips for doing this on a first play-through? 
 
Thanks for any and all feedback/advice :) 

OL spam Rain, while controlling Raven, and hope allies break his guard.

I set the difficulty to Easy (not sure if this actually helps but I think it makes guard breaking him easier).
 

Yeah, this is one of the hardest secret missions. I went with the same method as Mcfart and OL spamed Rain. Couple of extra pointers:
- Set the game mode to easy; guard breaks happen more often
- Set your other party members to do nothing (you don't want him to lose HP to quickly)
 
I tried using the method Bigandtasty suggested, but it never worked out for me. Really this secret mission comes down to luck.

Honestly though it really depends on what you think of the game overall. I missed out on the second secret mission only on my first play though, but found the game fun enough to play through a second (and third!) time to not only get all the secret missions but all the achievements as well. This secret mission took me several attempts both times I got it.

It took me a couple of tries to get it.  Just take control of Raven and spam the attack.

Hey guys, thanks for all the help! 
 
I actually did this last week by just spamming Rain with Raven and getting lucky (after like 15 gawd damn tries!!!).  I never even thought to put the game on Easy Mode, but I also don't feel right by doing that :p  
 
This Secret Mission I hope I will never have to do again, as I believe I'm on track for getting the Title with just last 3 Secret Missions left for me to do.  Thanks again!