Second on the Archaeologist, its a great dip especially if using a whip, and it holds up pretty well up to 6th level if you want more of it. Make sure to take Lingering Performance, so each of your rounds of luck can get tripled in duration. I don't think Greater Whip Mastery is worth a feat, but that comes so late probably doesn't matter.
Pain Taster looks...really unimpressive, and Cruelty kinda overlaps with most of the benefit of the Whip Mastery feat. All the class seems to do for you offensively is +1d6 at 2nd and 4th level sneak attack-like damage except without any restrictions (immunities, range limits, etc...) at all. That's ok, but not that amazing. If you do go for Pain Taster, definitely consider the
Unbreakable Fighter to get Diehard and Endurance for the cost of your 1st level bonus feat (and the Bravery replacement is just plain better, too).
I'd also suggest you get the
Fury's Fall feat, since you'll probably want good strength and dex...or were you going to do a Finesse build?
Also worth considering, if you plan to do tripping, is 3 levels in Inquisitor. That will both give you the bonus teamwork feat to snatch
Tandem Trip with, and more importantly, the Solo Tactics class feature to get the benefit any time any other ally at all threatens the foe.
