What do you mean by "primary tenant" and "cotenants"? If you're all on the lease, this has to go through your landlord. If the "primary tenant" is the only tenant on the lease with the owner of the property, then the "primary tenant" is landlord to the other roommates, who are subtenants. Only the landlord can evict a tenant.
The roommate who was assaulted can try to obtain a personal protection order, forcing the offending roommate to move out even if he still has rights under the lease.

