It's a little odd. Did you write it? 'Arm himself' is OK, but stretches the meaning a little. To 'arm yourself' normally means that you carry a gun, or some other weapon. So, perhaps you could say that getting an education is getting a type of weapon, to help you in your battles. But, we would not say 'become a great'. It should be 'become the great'.

'Arm yourself with knowledge' is a fairly common expression in my opinion. UK based native English speaker here.

He will thoroughly prepare himself to become a great accountant he dreams of being.

it means he provides himself with knowledge