I Try my best to explain
In terms of being underleveled : A lot of people go back to Blade And Soul Gold starter/beginner zones so they get an edge for PvPing that may be the reason, also, when you get to the first city (i think its jade or bamboo village (not sure for that matter) you should be around 15-ish 16-ish
Speed of leveling : Define leveling taking ages, if i make a new character and ignore PvP and just focus on speed questing i mostly hit lvl 15 in 2 to 3 hours.
Combos and gameplay for BM : This is really for all classes to apply : There is no suited ”how to play” way simply because everyone does have a different play style and with soo many options how to open-advance-end combat that its really not needed, also the game provides some basic combos in the class tutorial / quests , also some classes are more combo heavy then others, with Assassin and KFM leading and BM/SMN (note. this may not apply to everyone since i never really got into those 2) being at the end (They still can put up a lot of combos) also this game is HEAVY player skill/ping oriented in the 1 vs 1 scene soo it really depends how good YOU can be, and no guide will help you ever there, since it may say you : Use skill X when Player Y is using skill A. and there is the problem, the combat is soo fast paced that you not really have time to know what skill is being used (oke trolling Assassins while they use stealth in hopes to get you, and you all of sudden stop attacking and they just waste it and you see them looking at you with the : Well *cricket* : face its really worth it xD )
Also a good way of practicing combos is to do Dungeons Blade & Soul Items solo and practice on the bosses (you can train : Blocking timing, Evansion, How-to Burst damage between boss animations.
Glide/Flying : Dunno, but i think im quite sure they wont get faster.
I hope this at the least awnsers some of your questions.