I made a semi-successful summoner and here are my thoughts:
Being a true summoner, i.e. having your minions do all the killing for you, is not really a good idea. You receive less experience for the kills. So take time to invest in at least one powerful attack spell (lighting, elemental strike...).
You'll want to start off with summon skeleton. As soon as you get skeletal wall available, however, sink your points into that. They are much tougher. Also, you'll want to invest in a powerful dreadknight when the time comes.
Master summoner is a must, as it boosts the levels of your minions. The transfer powers skill is debatable, because no one really knows exactly how it helps your minions.
Another summoning skill to invest in is resurrect. The skill is only really useful at level three and up (they die too easily otherwise).
The two shield spells (aura of guarding and spellshield) can be cast on your minions as well as yourself. Be sure to do this to help them out.
Aura of command is defintely a must, so the creatures will follow you. You don't want to have keep summoning new minons every ten minutes. Here's the bad news: when aura of command is cast on a dreadknight, he loses his powerful red lightning attack. This is a bug. There is also a bug with aura of command, but this might work to your advantage. Once I had ten minions following me!
Some other thoughts to help your minions out: The skills withering curse and curse, limbs of lead, and freeze.
If you try all of this and still find you're having a hard time, allow yourself one death scorpion from the deadly gift skill. Once you cast aura command on him, he will probably stay with you for the whole game. Try to stick with only one, however, because they are way overpowered.
Good luck with your summoner!
