One very easy way to balance guardians is to make it so that when guardians die in a raid, it cannot be used again for that entire raid.
For example, this will automatically balance Donkey’s short cooldown and fastest speed as one of the most common guardian. The game has to have some sort of risk/reward ratio in the gameplay. For a guardian to run so fast, with such short cooldown, if the guardian is ‘killed’ before the duration is finished, it cannot be used again for the raid. A counter measure to kill the guardian can then be implemented in defense. Otherwise, what’s the point of ‘killing’ the donkey if the cooldown is so short? With this implemented, the logic would follow like most games, that the fastest character is always the most ‘fragile’ one. Thus, the player can take the donkey and run around with max speed, but he has to make sure it doesn’t get killed for running too far into an enemy’s base or he cannot use the donkey again for the rest of the raid.
This will undoubtedly make Guardians’ weaknesses more relevant in the game.
If this is implemented, then it will give the opportunity to balance out “G-5000” too, as the only EXCEPTION to guardians being killed and not being able to be used again. “G-5000”, being a legendary-grade guardian, has the ability to auto-reconstruct itself after being ‘killed’.