Short answer - equip the highest initialive unit with the ring.
The long answer - if you want to use damage spells, you will need specially prepared unit for it. This means you will want a lot of spell mastery and spell damage items and enchantments on one unit, overwise most units are better spending their turn fighting. So you probaly need several mastery rings, ametists and cedarwood oil. Look for other possibilities to increase mastery, spell damage and initiative. Keplin has stuff with mastery bonus, keplin and archon have initiative bonuses from shards. I beleive tandis have something to do with mastery as well.
