From the Japanese tangled song ("Healing Incantation"), the lyrics go like “傷を癒せ、運命の川”-Heal wounds, river of fate I don't understand why 癒せ is used instead of a proper conjugation, is it a colloquial way of speaking? or is it a form that isn't used a lot?
癒せ is a regular conjugation. It is the imperative of 癒す, "to heal (something)".
Compared to commands using the て-form (which are often followed by ください), the imperative sounds rough and can be very rude. For example:
As for the conjugation:
Godan-verbs: Change the last u into e. (行く -> 行け)
Ichidan-verbs: Change the last u into o. (寝る -> 寝ろ)
する and 来る become しろ and 来い (こい), respectively.
For the negative conjugation (no matter if ichidan, godan or irregular), just add な to the dictionary-form of the verb (来るな! Don't come!).
Edit: To fully answer the question in the title of your post - "癒せる" would be the potential-form of 癒す, but here, it is really just the imperative that is used.