Difficult level... I'm going to suggest something probably not very popular, but which I have find surprisingly useful.
Mind Fog
This spell does no harm directly, but the huge -10 penalty to Will saves makes the target an easy prey for your next spells. As such, it is always used first for a deadly combination, or to make high-will casters easier targets for spells with will ST.
The drawback of this spell is that... it actually give a Will ST itself!

So again the casters are still having the edge against it. However, it's an AREA spell and as such if you catch 3-4 enemies inside you're pretty sure that at least some will be affected.
Prying Eyes
It's only a scouting spell, but it really helps a lot. If you have darkvision or can afford improving your visual capacity with other spells, it can be enhanced a lot. The long duration, relatively long range and multiple eyes let you use this spell both for scouting an open surrounding area or a dungeon.
Drawback: the biggest limitation is that it cannot open doors or bypass physical barriers. Also it takes time to send the eyes around, so it's not for emergency use. The eyes can also be noticed, but typically it's not easy.
Wall of Force
It works only as a short-duration defensive emergency spell, but basically it's unbeatable (Disintegrate is the only common mean to destroy a Wall of Force). If you are surprised and you need time to buff up, or if you just want to run.
The drawback is that most of the time you rather prefer to smash your enemies

. If you could shape the wall - but you can't - it would be more versatile but not very, unless the duration was also much longer.
There are many spells at this level also which are very useful. Most commonly seen IMXP are not the ones I proposed but rather
Feeblemind (anti-caster spell),
Baleful Polymorph (actually better than the previous 99% of the times) and
Teleport (which may be required less often but also achieves game-breaking results). These however depend a lot of which spells of other levels you have prepared, because their effects (insta-kill the first 2, teleportation the other) are done by many spells in the list.