Rotten Penny

[Thunder7070]

Could someone please explain to me how I can unlock the Rotten Penny?
I'd also prefer it if you would describe multiple ways to unlock it. I only know that is has something to do with blue flies.

[Nazemec]

Guppy + high tear rate, soy milk for example. Sounds like something that could work.

[Fracastador]

I believe you need to have a certain number of simultaneous blue flies. I have not seen the exact number, since people who get it tend to make huge numbers simultaneously and not actually get a minimum count.
Using the guppy transformation or a black rune in an area full of items are probably the most broadly reliable ways, but there are lots of items that make blue flies.
I got it with guppy and a moderately high tear rate. Definitely higher or equal to 5. You just need to run into the right type of enemies consecutively to build up your fly count a bit.

[kone]

You need a lot of blue flies at once.

Compost item duplicates flies easily
Guppy's Head with lots of batteries or Car Battery
Black Rune a room full of pickups
Fish Head + lots of health willing to spend
Jar of Flies
Guppy

[RocketPotatoL62]

Using jar of flies at full capacity was all it took for me.

[Fracastador]

Originally posted by Sonic_RocketPotatoL62:

Using jar of flies at full capacity was all it took for me.

Forgot that one existed.

[Vinnyvinvinvinvin]

All of these are suggestions, though I got it by picking up items that replace all of your health with something else (abaddon or brittle bones) as keeper. Since he can't have any other health other than his coins, he dies with enough flies to get the achievement

[Lucathegreat]

I just got it on Greed, I became Guppy and had Guppy's Head as active item, just buy a ton of batteries and keep using it to spawn more and more flies

[Thunder7070]

I got the achievement via Jar of Flies. Thanks for all of the advice in this thread.