Legendary Guardian is a good mastery, but... [Gameplay]
it's not a healthy one.
Riot, you guys have the Offense branch down pat. It feels meaningful to invest in that mastery tree as most champions. But as for champions that don't want to invest in more damage, their masteries are unsatisfactory.
**Rework Legendary Guardian. **
No one is wishing they had more flat armor/MR come lategame, except for people who don't want to build any of it who might as well put 21 in the Offense tree instead. More flat resistances are for laning phase - masteries at the bottom of a tree shouldn't have that. But even if Legendary Guardian increased your armor/MR by a percent, or was just straight-up damage reduction, it wouldn't be healthy. It deserves to be something tanky champions can make good use of throughout the entire game.
i.e. Legendary Guardian: Increases your Cooldown Reduction by 1/2/3/4% for every nearby enemy champion.
There. Doesn't that make it a lot more interesting of a mastery? Doesn't that incentivise diving for champions who already want to take hits for their team?