I only use platinum armors on units I want to take past level 50. Those are much harder to get than the 100k gold or whatever that it takes to get 40 to 50. Plat armors should never be used unless you have 3 fairies of the correct rarity for the unit that is being fed to take advantage of the full 8,000 exp they can give. The guide you were reading was clearly written by either someone with a different perspective on how they should be used, or someone who was trying to prepare you for castle retake.

It isn't necessarily a bad thing to use them to level to 50 for the first time, but it certainly isn't necessary. There isn't really anything to rush ahead for the in the game right now, even the current event rewards are kinda bad. And I don't think you can do all of the event by the time it ends anyway. Just take your time and level up Nanaly, then a healer, a witch, an armor, a mage, another healer, and then just whatever you feel like after that. For now, until you get a large amount of CC'd units on your team, level your units to 50 then CC and re-level to about 35. Once you get something like 6 CC units at that level, you start noticing how much easier things are getting.