I have, though a bug may have been involved. Stocked up on health potions (equal mix of +100 and a lesser one in case they needed topping off but not the full 100), -enemy pow, +focus (I needed WAY more than I had, so get plenty), and +team pow. Team had two heroes with loads of health and two with lots of damage. (I don't know how others level their heroes, but I tend to focus mine on one or two stats, with different heroes having different combinations. For example, Galaxy Girl is nearly pure health, Hothead is nearly pure damage, @man is all about the intelligence, and Gloomy is a health/int mix.)
I bring up Gloomy because the rest of the team died when Beastfly at half health, I was out of potions, and most manager abilities were on cooldown. Defeat seemed inevitable. Perhaps I should have paid more attention to the fact that Gloomy was still at full health despite her teammates being dead... Monster hit, Gloomy's at half health. She swings, her health pops back up to roughly 75%. He hits her again, she drops to 30%. She brings herself back up to 60%. He stops to roar his -pow ability. Wham! She uses the opportunity to regenerate her health to full. Rinse and repeat, for the next fifteen minutes or so! Beastfly kept to his attack, attack, -pow rotation, which allowed Gloomy's health regen to just barely keep her standing despite a few heart-stopping moments. Her regular attacks aren't that powerful, but she whittled him down with me cheering her on and hitting manager cooldowns every time they came up. I really wish some kind of +intelligence item had dropped, because it would have been her rightful spoils. (Was a +pow item instead, which went to Hothead.)
I really wish I'd thought to check her inventory at the time, (I tend to rotate items in and out based on what my heroes are up to), because I'd really like to know if I found a good combination for Gloomy or if the whole thing was a bug, albeit a glorious one.
So far as strategy is concerned, what worked for the first 50% of Beastfly's health was tons and tons of potions, possibly 50 of each, (I highly recommend having two different levels of health potions so you don't have to either waste an expensive high-powered one when your heroes aren't that low or agonize over whether they're going to survive the next attack.), having a team member with -pow or some way of healing the team, making sure team members' abilities are set to single-target attack (obvious, but if I had a coin for every time I've forgotten...), and really, that's it. There's no complicated strategy needed, and no surprise abilities. In my experience he follows an ability pattern, which means you can easily tell what he's going to do next. Keep whittling him down, and pop a potion whenever your heroes need one. Good luck!