Grixis Affinity floods the board with artifacts and cheap artifact creatures like Refurbished Familiar and Myr Enforcer, so Cast into the Fire's mode to exile target artifact is useful against key pieces like Ichor Wellspring or their artifact lands, while its other mode pings two 1-toughness creatures at once. Annul can't counter sorceries like Thoughtcast, Reckoner's Bargain won't be stopped (it's an instant, not artifact, so Annul misses it) but Annul still hits Ichor Wellspring, Nihil Spellbomb, and their Myr Enforcer/Refurbished Familiar threats before they resolve. Spell Pierce is weak since their affinity costs are already reduced and they'll have mana to spare; trim it along with a Preordain for more proactive answers. Your Fiery Cannonade already answers most of their small creatures, so keep that at full count.