the tonberries, like all other monsters in this game level up with you, so you could say you were relatively high levelled :]

What you need to concentrate on is getting better magic to max out your junctions rather than levelling up. Levelling will probably only hurt your chances. Get some good ammo for Irvine's limit, get the best gunblade for squall you can, and stock up on aura and just pelt them with limits.

If you do level, then level your abilities in the cactuar desert (you have to walk a long way, but you can make it with the garden) so that you don't actually, you know, level up.
Level up your gf abilities so you can refine magic so you can junction it and kill tonberries 8)