100% melee damage is 20 base damage with beelzebufo sweep attack = 60 torpor over 10 secs. A second sweep attack within that 10 secs will only deal additional damage. 1000% melee damage will do 600 torpor over 10 secs with sweep attack :)
These oversized toads are easy to tame. All you need is a bola, tranq arrows and meat to tame them. Their food drops faster than their torpor, so you won't need that much narcotics. Be careful not to tame one if there are others nearby.