I believe all those shield skills are useless. They do well against human sized enemies, but it's usually not necessary, because you can kill everything with your weapon anyway. And blocking the big enemies is often not an option.
I mean, most enemies are such pushovers there's no need to block them and hitting them with your weapon usually does more damage anyway. Demonspite was nerfed a lot, so the damage isn't as high as one might expect of such a skill. Holy Wall...no idea. I can tell you it doesn't do what it claims to do in the skills description, but what it does is beyond me. I always thought it's just a giant shiled that will cover my whole party, but it doesn;t do that at all.
The team buffs are great, but they're weaker then the normal kind of buffs and don't last as long. They serve well if you want to teach your pawn to buff you though, especially in strange situations as mid fight etc.
If your weapon has, let's say Fire and you buff it with Ice, the Ice will overwrite the Fire innate buff and adds it's Magic damage to it's own, resulting in very powerful Ice buff. It doesn't strenghten the buffs secondary effects, like stuns, heals, etc. though.
Great Cannon is a great skill, but as I said, it has horrible vertical homing, so in most cases it completely misses flying enemies. Harpies and such are ok, but you have no chance in hell of hitting a Dragonkin, Gryphin, or Strigoi with it. Staff Bolts to a much better job, but usually lack range (depends on buff though) and stagger potential to bring the enemy down. I you fight an enemy without a weakspot (the skill targets random body parts, so it's unreliable on enemies with weakspots), let's say a Chimera, planting two Great Cannons and hitting both of them with melee (Full Moon Slash works wonders) results in devastating damage.