Well, Seraph is Dev. Controled so you would have to expect some dev magic regardless of what level he is
all he has to do is activate his dev invulnerability powers and ur as good as dead. so unless he wants you to win you wont.
well they won't activate their invincibility power unless they have to, they usually have Dev enhanced clothing with like 10% acc and stuff on each peice.
on some they have 5%.
There is a rumor that Seraph can only be beaten by Data Miner... but well... you know... Data Miner ownz all so it might be a sploit. Seriously... the NPC is way higher than you, if you solo him you would use alot of pills... then there is the dev/LET version... and I doubt they would ever let someone beat him.