I am attempting to create a powerful arcanist preoccupied with ridding the world of evil in general, fiends in particular, and demons specifically.
What are some good ability/feat/spell options for just such an arcane demon slayer?
You may want to consider building a large part of the character around casting quickened dimensional anchors. Pretty much all the biggest baddest outsiders teleport at will.
Take Wizard school as your first Arcane exploit, Focus Abjuration. Your choice of opposition schools is up to you but enchantment and Conjuration are needed so do not choose one of them. Take spell focus abjuration, spell focus conjuration, augment summoning, spell penetration, and heighten spell (quicken spell at higher levels is also a good option). Other exploits should include spell tinker and (IMO) the bloodline exploit celestial blooded. Items to get throughout your adventures should be a Ring of Archons, arch mage robes, Staff of Abjuration (or a staff of the master crafted for abjuration would be best). Banishment spells are of the abjuration school. Abjuration also includes spells liek repulsion and globe of invulnerability which can make you imune to some of the effects of demon's spell like abilities and/or the spell casters who summoned them, as well as making sure pesky melee attackers cannot approach you. It also includes dispell magic and remove curse (no-brainer against evil outsiders). Utilize planar binding to summon Archons for difficult encounters (divination would also be useful to foresee potential dangers ahead)
Note: banishment type spells do not allow a save, you only have to bypass spell resistance. Which you can do with spell penetration and items, as well as your arcane reservoir. Favored spell (dismissal) is another good feat if you want to dip in cleric or paladin for some possible mystic theurge play.