Though the character sheet shows a decrement to class levels, this only impacts regular spells that check the caster level. If the class level is checked instead, or if feats make use of the caster level, then level drain will not have the caster level penalty. Spell effects created by a creature under level drain are just as difficult to dispel as from a creature without level drain. Level drain can be removed with restoration (or greater), but not with lesser restoration.