@Red225
Before I begin, don't take it too seriously. It's difficult to consistently power-gauge all characters, and most of the time balancing magic techniques are pure author BS and plot conveniences. I also agree with you that burying him under earth might work, since the shield seems to require the guy stand still. However, considering the dude can use destructive thunder but doesn't have the strength to snap a neck, I wouldn't treat the power balancing too seriously. That said, I can think of a few reasons people look down on earth magic (especially when it's a flat plains environment, AND the combatants can FLY).
In my personal opinion, the smartest thing Frey could do was make the ground uneven with magic (unless his main magic capabilities are limited to fire and ice, which renders everybody's arguments moot), and then hide his allies in cracks underground while making stone dummies. Another possible reason is if he were to hide them underground, then the Apostle can bury his allies easier (since they're already sheltering in the ground).
The main reasons why earth magic is usually looked down on by mages for plot reasons are: speed, durability, and variability. Speed is because compared to fire/wind, earth magic usually takes the most time (take a chunk out of the ground and manipulate it before you can even get to attack with it by USING WIND MAGIC TO ACCELERATE IT at the enemy), the fact that massive explosions with fire/ice would destroy regular dirt/rock anyways so you might as well use pure air and thermal energy instead (possibly also air pressure), and finally the fact that unlike all other forms of magic, earth magic HAS to come from the ground. You rarely get earth magic coming down from above (stuff like meteors, which even then are always either super-slow like the massive stone/earth spheres, or basically fireball magic instead and not really fitting "earth magic").
Basically, earth magic sucks unless you're fighting underground... in which case it's the best magic to have possible (for the same reasons it usually sucks).
Also take note that naval/sea battles usually rely on pure ice/water magic, and occasionally wind magic. Nobody uses earth nor fire magic in a sea battle, for the exact same reasons as above. It's simply a question of environment influencing battle decisions, much like deciding where to step to force your opponent to have unstable footing when fighting on snow layered on ice, for example (I did dumb stuff fencing a friend on ice in the winter once. It was fun though, since we were so bundled up everything we did was safe).
Anyways, the only reason why I cared about the magic balancing at all is because I'm setting up a story of my own and want to use an extremely hard-magic system limited to close-combat magic techniques, mainly limited to kinetic manipulation and (technically) indestructible magic weapons.