Picking what to boost
The criteria (in order of importance):
- Deal 6k+ damage to high defence monsters (or damage is not important because they are for support, disruption, etc.)
- Generically very strong
- Mythics
- High defence
- (Bonus) Large impact on the first turn, or benefit from surviving (extra HP helps get there)
Not many monsters meet all criteria, so the bonus can make up for some shortcomings in the others. It’s most important that the monster deals 6k+ damage than it being high defence itself, because if you’re facing a meta of high defence mythics and your monsters can’t kill them they’ll be too easily ignored while your monsters which do enough damage are targeted and killed.