Casket of Fury

The Reaper doesn't appear early, but its set of abilities remains relevant throughout the game, and I found it to be the most successful and effective. Dealing percentage-based damage is always a strong ability, and it scales well with the length of the game. Black Hole also deals good damage; the damage is noticeable until the very end of the game, which cannot be said about the magic of other spirits. Time Warp is another useful feature of the Reaper; it can be used in many situations. And Rage Gathering is useful if the battle starts with a minimal amount of rage and you prefer another spirit.

Towards the end, I only used Zeroka for the wall, to give the archers a chance to fire one extra shot. It sometimes saved the day, especially in combination with Linna's wall. Slaim has a great ability – Glutton's Armor; it effectively allows you to make one attack without retaliation, while also removing the retaliation. And it consumes very little rage (as far as I remember). And in the early game, "A Terrible Mistake" is an incredibly useful ability, allowing you to damage a large part of the enemy army, if not the entire army.

With Linna, I only used the orb and the crystal wall (I forgot the name).