Hello Yenaver, we are gonna need some proofs about that so we are gonna be waiting for that video. Besides that having MR set aint gonna resist all the time I mean gunner might have a second set with MA / MB for higher magical accuracy in order to be able to kill you and any other using MR set, as for the Sin (because I'm playing sin) they have really high dmg even with starter gear and if you socket the right manastones and play right your skills trust me the dmg is insane also a lot of sins in the server are playing with secondary godstone DMG and gunners most of the times on both guns so that's another one point for dealing a huge amount of dmg.
P.S I would like to mention in case of the sin that using MR set is just gonna Resist hes magical skills so he is still gonna be able to stun you and also the DPS will still be high, same if you use Block set he can still manage to deal you a huge amount of dps since sin's are based more on attack (weaving) than skills there for attack is countered by Phys def while physical skills with block also crit strike is countered by Strike resist, having a good knowledge of what deffensive stats counters the offensive ones you will get a much more clear picture!
As I said before tho, we are waiting for that video and we will keep that in mind.