All of the powers were "made up," though I'm sure there are functionally equivalent powers. I based the average damage values on similar monsters from the MV, like the level 8 elite brute Owlbear. Of course, as a solo, the monster needs a way to constantly threaten the party outside of its turn, and that's why I designed it the way I did. I did not directly steal any power (aside from Stand the Ground) from any particular monster or class, instead I followed the guidelines for creating challenging solos as per my understanding of them.
As for why the creature was nearly fatal, I'd say it was a combination of bad luck, a few risks they took that ended up backfiring, and being intended as a very difficult challenge. One character only survived because I rolled rather low on the damage dice, so I'd say they had luck swing both ways. Solos have an interesting balancing act to manage well because the PCs each get a turn for every one of the solos turns, it's difficult to make them a challenge that's just the right amount of hard, and I feel this one was rather successful at it.
I can see where it would be made up and yet I can see where it would be "taken" from a similar power because mainly what makes them any different would be the fluff and a few bits here and there. Since 4th edition is mainly based on the numbers then I can see how "easy" it is to create a monster because all you did was look at similar powers and borrow the numbers from it.
It's like having 10 different numbers in a basket and you shuffle them around to make a different set each time.