For strength and immovability:
* As solid as an anvil - emphasizes unyielding strength and stability.
* Like an anvil, he stood his ground - emphasizes unwavering resolve.
* His face was set like an anvil, unyielding - implies determination and a lack of flexibility.
For heaviness and weight:
* The weight of the problem felt like an anvil on his chest. - suggests overwhelming pressure and burden.
* His heart sank like an anvil in the ocean. - indicates a feeling of hopelessness and despair.
For being unyielding and unforgiving:
* He was as unforgiving as an anvil dropped from a great height. - suggests a lack of empathy or remorse.
* Her criticisms were like an anvil hitting a nail, blunt and forceful. - implies harshness and a lack of subtlety.
Remember, the best simile will depend on the specific context and the quality you wish to emphasize.