monstrous vs grievous

monstrous and grievous both are adjectives.

Word	Noun	Adjective	Verb	Adverb
monstrous	No	Yes	No	No
grievous	No	Yes	No	No

As adjectives, monstrous and grievous are synonyms defined as:

Other synonyms of monstrous include atrocious, flagitious.

monstrous (adjective)	grievous (adjective)
shockingly brutal or cruel	causing fear or anxiety by threatening great harm
abnormally large	shockingly brutal or cruel
distorted and unnatural in shape or size; abnormal and hideous	of great gravity or crucial import; requiring serious thought
	causing or marked by grief or anguish