Summon Mages can go Wand, Staff, or if they feel brave, go Magic Knight. But if you're having troubles, let's stick with the classic pure summoner.
Earth Mastery III + Summoner III = Iron Golem dominates. The rest is details.
Skills:
Iron Golem Maxed (Earth Mastery + Summoner): The big guy destroys everything.
Staff Mastery Maxed (or as high as it goes): The high damage output pairs well with the Iron Golem as a walking tank.
Mage Armor + Mage Barrier: The extra defense, and pushback makes these two very helpful for a mage caught in a corner. Even if you opt to avoid a heavy investment in these skills, Mage Armor Level 1 will give a nice protection from a random trap (trap does no damage and disspells the armor.)
Fire / Ice Mastery: Player choice. Experienced players might be able to skip getting either Fire / Ice Golem as they rush to get Earth Mastery. However, new players, or those seeking a more well rounded build can invest a few points into one or both.
Note: Fire / Ice golems will be used primarily as a back-up to your golem, or in specific situations (let a fire dragon breath fire on a fire elemental all day long). You can get summons scrolls for them pretty easily by mid-game so not really a pressing need long-term to have these skills.
Traits:
You need to meet the trait requirements for the above skills, and will have heavy investment in INT and PERS. But beyond that, you slip a few points into END or AGL to help stay alive. STR is a waste unless you go Mage Knight, which is not the build being described here.
BIG NOTE:
Most players that struggle with a Mage Summon build struggle on the fighting style, not the actual build. Let your Summon and Companion take the vanguard. You're going to stay back where it's safe and attack with powerful range attacks.
Companion:
Hirge, with a shield, and healing as the only active skill. Hirge is bascially a HP source for your mage and summon. She doesn't need to do damage, just run interference so things don't attack you. You go go a full defense Griss but you'll do better with Hirge.