The best offense... is apparently a really high defense.
Alright. If you're building around this, there are two things you need. Defense buffs and readies.
The core defense buffs are...
All three are needed if you're building around this.
Specialized Training can be worth considering for an extra boost too since this will help you thwart it. (Although it's best in multiplayer with a Justice player.
Next, you need to decide if you're going to use allies or no allies.
No allies gives you access to Ronin for higher defense and Army of One for readies.
BUT there are readying cards for different traits that require allies to work.
Avengers decks have access to Earth's Mightiest Heroes which readies you by exhausting an ally. Web Warriors have Spider-Man who can provide readies whenever he activates. X-Men have Utopia which lets you ready when you play an X-Men ally.
Consider these when deciding whether to go Ronin or not.
Other sources of readying include CITT for Guardians, Reboot for Vision and Limitless Stamina for characters with 14 or higher base HP.
When every good option for readying is exhausted... we also have a bad one...
Let's talk Tenacity.
Tenacity is a terrible card under normal circumstances, being a massive 4-ER investment in total. But what it does have going for it is being able to trigger it at any time you want, which actually works well with this type of strategy.
Let's say you have a hero with 3 DEF. On turn 1, you play Innate Reflexes, raising it to 4. Now you can lay down Tenacity, and then when you play The Best Offense, your DEF raises to 5. Trigger Tenacity and then either attack or thwart for 5. That's a pretty decent return. It's hard to say a card in Protection that's essentially "choose to either deal 5 damage or remove 5 threat from a scheme" is bad. And this worth naturally increases the more upgrades you have. If you have all your base defense upgrades, that's 7 thwarting or attacking with a 4-ER investment.
So Tenacity... and I can't believe I'm about to say this... might actually be playable in this one specific sort of deck. Not great. And still by far the worst readying option you have. But it is an option once all the others are exhausted.
BUT... even then, I'd only use it in the ally-builds since FNE also gave us In Harm's Way which attacks AND thwarts equal to your defense for the same ER investment if you're going Ronin. And then the problem with the ally builds is that they do require so many card slots for allies that there's rarely room for something like Tenacity. But if there was ever a good deck for Tenacity, it would be this one.