Not sure if he will really be the right choice for legendary. In general defense stats are more important, check the Damage Calculation thread to understand how it works and why having a higher def stat than the attacker’s atk stat has a major impact on how much damage is assigned.
As you will use him as a utility hero, the right path would be def+HP, cause he will never do any good as a damage dealer on legendary or even epic.
But if you have struggle beating those stages, maybe show us your roster so you can get advice on how to set up the right teams