Sure, the guardian isn't overpowered, but it isn't that much worse than, for example, Solar armor's bonus. That gives 30% DR, but it doesn't stack additively with other DR bonuses, and you don't get to use it if you're being attacked by more than one or two opponents.
The guardian's only purpose is to keep enemies off your back while you or the dragon get around to killing them, and at that it works wonders, even if I forget to use it a lot of the time.
I will admit that it's current use is a bit niche, but to solve that, I would give it three 'modes'.
Sentry: Exactly what it does now. It stays in one spot, and keeps enemies from attacking you with knockback and aggro. Maybe slightly longer range, but this is far from necessary.
Assault: It follows you, and attacks enemies with single-target homing energy bolts dealing 100-200 damage. I imagine this would be the least useful, since summoners don't need more damage, and often you can't even get in more damage with an actual weapon, let alone another minion.
Protect: While not in the other modes, it stays near the player and gives a defensive buff, on line with the Dryad's buff, giving a bit of defense, and maybe some health regen.
If this change were made, the guardian still wouldn't be overpowered, but it would be useful when you're on the move, or just forget to use it properly.
