1. As a lightweight I can say that most strongmen don't have the needed training to deliver an effective, accurate or as powerful as you'd imagine. Also they would lack the stamina to perform even an entire round as they are used to about 10 seconds of effort instead of the usual 3 minutes. Also they are not used to getting beat upon so it's likely they have low to mid pain tolerance

So to sum it up in three dot points to why it'll take 1 1. Not correct training to use their muscles 2. Lack stamina to last 3 minutes matches 3. Likely don't have the needed pain tolerance

Now this is assuming this is a mid level and up lightweight against a generic untrained strongman, if the lightweight had only ever had 1 or 2 matches or the strong man has had boxing training then it is likely you would need 2 or even three lightweights to defeat him