XP packs is not necessary at all, but can help with faster exping.
Game system using "experience penalty", while you fighting against monsters which no have you level, in both directions - lower, higher. Yes, in case that level is higher it is total nonsense, conflicting with all RPG rules, but this system is from original game made by Flagship. (I suppose it was prevent quick leveling for low level players in online game, ingenious thought
)
So, if you have problems while gameplay, try farming xp in some level where monsters is on your level, or in level where level of monsters is always same like yours (Piccadilly Circus - Beral, Southwark bridge - Rashaverak, Trinity Square - Katamari ...)