Question about simulacrum

[Psyren]

2) For its special abilities, if any are way beyond what other creatures of its new CR (half) have, then weaken them. This takes some winging it.

This is the important part.

3) If an ability is too strong, but it's not clear how to reduce it without changing it completely, you can keep it unchanged and add a 50% failure chance.

Keep in mind that for at-will abilities a failure chance may not matter, and also keep in mind the bit above.

Create a Simulacrum of a 36hd Pit Fiend. That way when it's halved it's the normal 18hd Pit Fiend.

If your GM creates an advanced Pit Fiend for you to copy then go for it.