"He is impossible to beat."
Vertaling:Hij is onmogelijk te verslaan.
I believe the "om" is unnecessary unless there is at least a piece of a clause associated with the verb (in this case "verslaan"). For example, you would include the "om" if you were translating "It is impossible to beat him" because you have the phrase "to beat him". You are not associating just the primary clause (it is impossible) with a verb (infinitive) but with the verb phrase. Is it the beating that's impossible? No "om". Is it the beating of something that's impossible? "om."
Hij is onmogelijk te verslaan. --- the beating is impossible
Het is onmogelijk om hem te verslaan. --- beating him is impossible.
And if you had a sentence with a modal verb, there is no "te" and no "om ... te."
Je kunt hem niet verslaan. --- "can't beat him."