If you have a Fleecemane Lion that is enchanted by Shielding Plax, and you pay to activate the ability Monstrosity 1, then would the creature fail to become monstorous and would not receive it's +1/+1?
Monstrosity does not target, so it is not affected by Shielding Plax. Something is only a "target" if the spell or ability uses the word "target" in its text, or the rules text for a keyword ability, or if it is an Aura spell, which is always targeted.
114.10a Just because an object or player is being affected by a spell or ability doesn’t make that object or player a target of that spell or ability. Unless that object or player is identified by the word “target” in the text of that spell or ability, or the rule for that keyword ability, it’s not a target.
Shielding Plax only prevents the creature from being targeted, so it has no affect at all on things like Monstrosity.