if i was you. do the quests. but dont do them religiously... just get the ones corrisponding to the mob ur killing, i mean why not

"Taxi" is usually a term for a payed power level... where a high level kills mobs that are roughly 9 lvls above you and you get a huge amount of XP. this is only really good if the taxi has a very high level killer, preferbly a nuker or wizzard
a gap is the gap between your level and the highest mastery you have
ie
level 26
fire 24
light 22
bicheon 18
your gap = 26-24 = 2
sp farming is when usually refered to having a 5,7 or 9 gap where you kill/level up/grind w.e soley for SP and not EXP. often sp farming includes dieing so you loose EXP but not sp. and repeat the process.
a nuker is a character who uses nukes.
nukers are... (ch chars who use nuke skills, usually cold light bicheon or cold light fire)
can also be referred to Wizards and battle bards
hope i helped