Melf's Minute Meteors:

Since you soloing, you cannot rely on a party member to wield a +6 enchanted weapon, because you have no party members. Melf's Minute Meteors can hit anything and deals decent damage. Thanks to its' elemental fire damage, you can hit Stoneskinned Mages( be careful of Spell Turning though) and finish off Trolls. It is also a very rapid attack with 5 attacks per round.

Skull Trap:

Use this as a trap, lay several of them and lure enemies into the area.

Haste:

This spell will greatly enhance your summons' fighting skills.

Flame Arrow:

Absolutely devastating in a Spell Sequencer. You get 1 bolt for every 5 levels of the caster, up till a maximum of 4 bolts. Each bolt deals 1d6 of physical damage and 4d6 of fire damage. So, at level 20 and beyond, you are dealing 20d6 damage per shot. Put 3 of them in a Spell Sequencer... and voila! 60d6 damage!!!! Note that the battletext can be misleading, the game bunches up the fire damage in pairs. So in reality, the damage seen on the battletext window is 1d6 physical damage and 2 sets of d12 fire damage per bolt. This spell is great against Beholders and if you are fast enough, you can destroy mages before they cast their protection spells.

Slow:

A slow target is an easy target. Use in conjunction with Greater Malison when you enter ToB.

Dispel Magic:

You have nobody to rescue, and if you are disabled, you are mostly screwed already anyway.

Lightning Bolt:

Too wild, to put it mildly.