I was planning on making one after I did the Guardian of the crystals, but didn't actually feel like doing it after all. In a nutshell the strategy for that fight is "spam shock and nuke it".
I'm pretty sure I was the only one who beat it how you would "traditionally" beat a boss by buffing my attackers and switching them in and out while healing my tanks.
Also pretty sure the accepted strategy for that boss is to give Rin Magician subclass and the main equipment that inflicts Shock and just spam Cat's Walk.
Edit: I just happened to read the wiki entry for Guardian of the Crystals. This is probably going to come out sounding harsh, and while I appreciate the contribution, I couldn't just silently edit this one.
"It also regenerates health each turn based on the number of crystals still alive, roughly 77k per crystal."
"Your best of bet of winning this fight is taking out the Guardian first, since it is by far the most deadly of them all and once it falls you can keep your party easily buffed up."
Like seriously? Kaitani, pray tell why should we focus on taking out the Guardian first when it regenerates health based on how many Crystals are alive? You even went as far as to say the Wind Crystal is relatively weak to Death but still insist that the Guardian should be taken out first.
The boss fight strategies shouldn't be written with Flandre cheese in mind because that's what it sounds like.
I'd rewrite this myself but sadly its been far too long since I fought this boss... From what you've written, it seems like the best way to go about the fight is to take out 3 of the 4 crystals leaving one alive before hammering away at the Guardian. That said, I have no idea which crystal should be left alive and my personal experience doesn't help either since I took out all the crystals and dealt with the boss by himself.