In addition to the already mentioned Power of the Spheres and Light of the Northern Star, there is also Summon hellpower (tricky, but power comes at a cost), also the various individual magic boosters if casting a non astral spell (ie, Strenght of Gaia for nature...) and finally, Communion.

There is also a spell that boosts penetration, through indirectly: it's Nether darts. Any dart that succeeds in beating its target MR will feeblemind it, and a feebleminded critter gets -4 or -5 to its MR, so any ufrther spells have a much easier time getting through. Seeing as a Nether darts casting throws a couple dozens of darts your chances are pretty good for one of them to succeed.