0 - 50: New Haven. In the 12 Mage's Guild shop, seek out the NPC, Alefian, to receive a quest that requires you to raise Resisting Spells to 50 through battle. At some point in the quest accelerated skill gain will activate which will result in the normal 0.1% skill increase increment to range from 0.2-0.5% lasting until the quest conditions have been satisfied. The title of the quest is Defying the Arcane and the reward is the blessed Bracelet of Resilience.
30 - 120: Hew Haven. Unarmed, target several Spectral Spellbinders. The more that are casting on you, the faster you can gain. Once, a damaged house to the South Southeast of the remaining Haven buildings allowed a player remain out of melee range while still subject to the Magery and Necromancy spells thrown by the Spectral Spellbinders, but no longer. However, if you venture North from, Old Haven into Weylins house, you can teleport into the corner of his bedroom, and be able to remain out of melee range, while the Spectral Spellbinder is casting spells on you.
Malas. "Hook" a Crystal Elemental on the front steps of a house. This will allow you to stay out of melee range while still subject to the Magery spells thrown by the Crystal Elementals. It should be noted that while Crystal Elementals cast the Teleport spell, creatures are no longer allowed to actually teleport and therefor will stay stuck on a house's front steps.
At Sea. Sea Serpents cast Magery spells and are slow enough to allow a sailor to gain Resisting Spells at a nice rate.
Magery. While training Magery you can also train Resisting Spells using these cast on yourself: First Circle Weaken, Fourth Circle Mana Drain, Fifth Circle Paralyze, Seventh Circle Mana Vampire