Beowulf

Easiest way to get this achievement is during episode 10. An objective will come up called "Kill the Formor Leader" enemies will now endlessly respawn until you kill the leader. Keep using combos and fill up your blue health bar, once it is full let loose Heroic Storm (+), rinse and repeat, do it 38 times for the achievement.

There are also other areas in Episode 11 (Kill the Formor Leader) where there are infinite spawns for you to take advantage of. Thanks to Epsilon Theta for the info

Heres a video solution showing where to get this achievement.

As the other solutions point out, start using Herioc Storm as early as possible during your Heroic playthrough to reduce the time it takes ot farm it.

I recommend you go for this achievement on your Heroic playthrough, assuming you want the achievements for both paths (Carnal and Heroic). It just makes more sense this way. Also, get started early, as this (at least to me) was one of the more time consuming achievements.

It needs to be pointed out that Heroic Storm only occurs only when your Heroic Gauge (the blue bar) is full and you invoke the Heroic boost (LB+B) command. If done this way your will hear a bit of music while you and your Thanes are in Heroic Storm mode. I made the mistake of thinking that if I could do LB+B, that was a storm... But it wasn't. Also, make sure there's no enemy near as your getting ready to do the storm, as it takes a second to power up, and if you get whacked at this time you'll have to fill the bar back up.

The other thing to point out is the more Thanes you have with you, the more it takes to fill the gauge. So doing this earlier in the game makes more sense (unless you take the cowardly approach detailed below). By now you should know it takes heroic actions to fill the gauge. I found dodging (LT+LS+A) and killing with the Y button worked best.

So now onto my method. You can take everything above and use it with Crimson Drifter's method. Sadly I missed his opportunity and had to find my own. The spot I found was right after you finished protecting the Holy Relic (that was fun, wasn't it?). After you lower the bridge and come to the next wheel to turn there will be a spot where 4 of those evil little fury critters endlessly re-spawn.

What I did first was turn the wheel and run up to Herod's flag to get my Legendary Sword and Shield. Then I went back down and started going Heroic on the little beasts. At first this did not go so well as I had like a dozen Thanes in my party and filling the gauge was slow going (and a single whack lowers the gauge dramatically). So I did something very un-Heroic and went all Carnal on my boys and thinned the ranks out (be careful not to whack Wiglaf). Now with only a few Thanes in the group I could do a dodge and a kill and get the gauge full (your Thanes yell "Hero" and all blue circles are flaming). Hold LB+B (even the B option looks a bit different now), hear the song, rinse and repeat. Once the achievement pops, let yourself get killed or whack Wiglaf, and you start over at the wheel again with all your Thanes intact. You can now do all the collecting activities for this area.

Not very Heroic, but this might be your last chance to farm for this achievement. It's kinda a tough achievement, because if you take a whack while filling your gauge, the gauge goes down, a lot... Very frustrating. That's why my little farming spot worked as the enemies are weak and always come from the same place. If you hide around the corner, in the light, you can usually get the drop (and dodge) on them as they come around the corner.

Good luck!

PS. Thanks to Crimson Drifter for setting me straight on what a Heroic Storm is.