Here's what rhymes with insuperable.

Consider these alternatives

insurmountable / accountable unsurmountable / accountable unbridgeable / double surmount / amount unpleasing / leaving unassailable / available surmountable / accountable unrealizable / advisable inconsiderable / considerable avoidable / voidable unmanageable / manageable unapproachable / notable unachievable / feasible unrecognisable / recognisable