Fusion means union, blend, i.e., a process in which two or more things join to form a new substance. So, isolation, which means separation or segregation, is its antonym.
Somnolent means sleepy, drowsy. So, its antonym is active.
Mellifluous means pleasingly smooth, sweet-toned. So, its antonym is discordant (harsh, cacophonous).
Prodigal means extravagant, wasteful, etc. Its antonym is frugal (economical, thrifty).
One of the meanings of disaster is the failure, so its antonym is the success.