I equipped the Undead Gem to Lilith and Fleon. (The Undead Gem makes it impossible for the enemy to "kill" you in battle - the worst they can do is reduce your HP to 1. However, there's a 25% chance of dying on every turn. I'll explain how to counter this below.)
First, you need to get Fleon's SPD ridiculously high. To do this, complete Sub Quest 15 enough times to get four SPD Ore 3. Put them ALL on Fleon's weapon. Don't accept anything lower than 85% for any of the four slots:
[img img=277]Screenshot_2016-02-10-23-30-09.jpg[/img]
I did this with Fleon at Level 311, and the result was that his SPD is 3531.
On Easy mode only, this SPD is high enough that Fleon gets his turn before Guild Hero.
You also need a minimum of 30 MILLION Gold. This way, when Fleon uses Gold Rush it will do 999,999 damage (which is the most you can do on one attack), three times.
When you have all that set, go ahead and challenge Guild Hero - but be warned... If Fleon's first turn does not come BEFORE Guild Hero, you might not be able to pull this off.
This is the SECRET to winning this battle:
On Fleon's first turn (which MUST come before Guild Hero's first turn), transform to Healer and use Beast Song. This skill prevents all status ailments for all allies.
This is essential, because Guild Hero uses "Final Inferno" on his first turn, which reduces everyone's HP to 1, and inflicts Poison, Paralyze, Fatigue, and Enrage. If you cast Beast Song before he can use this, all status ailments fail.
Now, most likely Caine and Julie will die before they ever get a turn. That's fine; just leave them be.
With Fleon's SPD being so high, he'll get a second turn before you get to Lilith's turn. On EVERY turn for Fleon, use Gold Rush (unless Lilith gets the 25% chance, and dies. Then use a Hi Elixir to bring her back.)
You need Lilith and Fleon to both stay alive, so if one dies the other can resurrect them, and meanwhile keep the battle going.
On Lilith's first turn, transform to Healer and use Reincarnate. This is a one-time, if a party member dies, they'll auto revive with 50% health. (This is how you avoid the risk of BOTH Lilith and Fleon dying from the 25% rule on the same turn, which does occasionally happen.)
I had to use Gold Rush 3 times, each time dealing the maximum amount of damage possible - 999,999. Also on Fleon's second attack (the second time I used Gold Rush), he got a burst break, and did a couple other attacks that each did about 5-6,000 damage. (So my theory is that, on Easy difficulty, Guild Hero has between 2.5 to 3 million HP - making him the strongest bonus boss since Apocalypse at the end of Alphadia. Only Guild Hero is much worse in many respects...)
After you win, you get the Google Play achievement (yay). Also, Guild Hero congratulates you, and invites you to come back any time for a rematch. ??? (Seriously, like I'm gonna fight you again?!)

[img img=278]Screenshot_2016-02-10-23-39-58.jpg[/img]
[img img=279]Screenshot_2016-02-10-23-40-04.jpg[/img]
I firmly believe this same strategy will work on Normal, Hard, and Hell - the only thing is Fleon's SPD has got to be EVEN HIGHER than 3531, or he won't get the first turn (and you NEED to use Beast Song before Guild Hero's Final Inferno.)
So, that's it for me. I'm gonna take things easy for the next couple days - I may do some grinding and try the harder difficulties; I may work on the Asidivine Menace walkthrough some more; or I may just take a lot of naps. :huh: (naps are good...)
If anyone finds another strategy, please post it here!
P.S. On Easy, all he drops is one lousy All Matter.Attachments:Screenshot_2016-02-10-23-30-09.jpgScreenshot_2016-02-10-23-39-58.jpgScreenshot_2016-02-10-23-40-04.jpg