When I attack either Big Foot or Elder Wrath, they will take 2 hits and while flinching i cannot hit them until they are done. While flinching all my attacks seem as though they do not register toward those mobs even though the mobs around them may still get hit by my attack. I don't know if that is just for my job which is Ice/Lightning Arch Mage. If any other job has problems with this issue i'm not sure.
All classes have this issue and it depends on weapon speeds. I believe you have to be within .75 second to be able to land all your attacks consistently. Anything faster and every 3rd or so attack will just pass through.
How can that be? Warriors sit there with constant attacks for days fighting BF; I see no unregistered attacks there.
For warrior it depends on the attack speed of their weapon. Heroes usually use a dragon claymore for BF so they hit 100%. ST heroes simply turn off booster and not miss either. However BF in v62 was bugged, as were many other monsters, like the elder wraiths.
Chain Lightning with booster on is .69 seconds which Bigfoot will be invisible to. Just attack it without booster (a rate of .78 seconds) and every hit should connect.