Carnage is a bloody psycho, Spawn knows the lunacy tactic when he fought Violator already. The differant is Carnage isn't as strong are as resiliant to damage as Violator, but he IS faster and can use his body as a living weapon.
But Spawn has the powers of hell and isn't afraid to use them, so I say Carnage gets a few hits in, but ultimately he's going to lose this fight. Spawn would do good to simply leave Cassidy a vegetable and not actually kill him less he want a bigger pain in the ass to deal with later..
Which may be to the demon's best interest if these two fought.
Because this means symbiotes are capable of fusing with the living. (A hellspawn's suit IS a Venom like symbiote..)