Obviously Star Trek changelings are quite robust.
I know the answers to some of these, but I'll just leave it to someone to provide a complete answer.
To harm means to kill or to do damage to a changeling that can be permanent or at least last for some period of time or may require medical intervention to repair.
Various means of attack could include, among others:
- physical attack
- physical separation into pieces
- extreme heat or cold
- energy weapons
- chemical attack
- biomedical attack
- denial of reversion to gelatinous state
- emotional abuse (jk)
In canon, what do we know causes damage to a Changeling, and what doesn't cause damage?