I managed to create some seriously overpowered creatures as well, however, I think that my creatures have turned out to be an example of how an overpowered creature can be easily balanced with just a few tags being removed or added.
What I did was create the Seraph and Fallen Seraph creatures they have the: ability to open doors, fly, go into trances, [FIREIMMUNE], [FIREIMMUNE_SUPER], [EXTRAVISION], [PARALYZE_IMMUNE], [trAPAVOID], [NOBLEED], [NOEXERT], [NO_DRINK], [NO_EAT], [NO_SLEEP], [NOFEAR], [NOSTUN], [NOPAIN], [NONAUSEA], [NOBREATHE], [SIZE:25], [DAMBLOCK:500], attacks that deal 4-36 BURN damage, [SPEED:50] = 6,666.666 in-game speed, and a heatdam point that is just under magma's temp, causing them to only be harmed by it very slowly, and they don't die of old age.
The things that balance out all of those tags are few, but mighty. Those things are the [NO_GENDER] tag, the fact that their entity is set to never send out adventurers, only to siege when you reach a metropolis with a population of 140+ dwarves, and most importantly, the lack of a [BUILDINGDESTROYER:?] tag, meaning you can just lock them outside if you wish or atom smash them. Also, they only like to live in caves, which slows their takeover.
All of this means that they are really overpowered, but there are only at max about two hundred of them, (I have their entity set to generate a lot, instead of just 7 per site), and since they never adventure, the only times that they will kill megabeasts is when the megabeast attacks them. If you do see a seraph in the legends though, you already knew who won that battle. I think that it has turned out as a wonderful example of how to create creatures that are balanced even though they are super powerful.