With leve allowances it's not that long to go from 0-99 takes about 12-13 days. As for xp boost from completing a leve it's not really noticeble til the 30+ leves where you can get a nice chunk of xp for completing them. Honestly I'd say if you don't want to get the xp boost to get to far ahread of her you could do that following.

I think the following works, what you could do is this.
-Stare a leve using your leve charge.
-Abandon your leve being sure she if in said leve.
-Then join back in leve with her
-Complete leve killing mobs focusing more on xp chaining rather than the xp at the end(aka if you're ina leve where you have to uncover a mob kill all the mobs before finishing the leve)
-At the end of the leve you'll pretty much both get the same amount of xp and better yet you'll be able to do the leve again without having to head back to town by simply selecting -retry- on the leve card.

From the 20-30 path the xp is like:
From leve mobs=5-10k
From completing a leve=1k

So really the boost for completing a leve isn't as great as it is to take your time, beat the mobs and chain on them

From what I've noticed the leve mobs also seem weaker than the outside world mobs so chaining them is easier then chaining the outside mobs. Since you do have someone else playing with you, you can easily dispatch foes that are 3-6 lvs higher than you whatever you feel more comfortable with and get a nice chunk of xp from it.