A cautious villain would've immediately Halocaust'd everyone without warning or even announcing his/her evil. Unless your goal is to instill fear in your name, if you're going for mass genocide, you'd try to get on the good side of any powerful faction and destroy from within.
I can only say read "Reverend Insanity" . He is a villain type and cares for none other then himself 99.99% of the time. The beginning is a slow start but once you get started it gets ya going real good. Maybe read the manhua 90 chapters or so first and then go to where the novel picks up
"Sir, the army is ready to attack."
"No it's not, we don't have siege weapons yet. Make some trebuchet's, siege towers, and don't forget to start digging tunnels."
"We have fifty-thousand goblins, and ten-thousand orcs. This is a castle of only five-thousand men. We can crush them!"
"No such thing as too much preparation. How are negotiations with the ogres and nightmares going? I want to recruit their aid too."
Watching the demons further fortify their army and position from a watchtower, I find myself wondering just how long this will continue. It's been five years already, since they first amassed their army. It's since grown from an estimated sixty thousand troops to over two-hundred thousand, yet my fear now is gone. It's clear we will all die if they ever attack, but after waiting so long and watching the army grow and the massive siege engines rise up... it's just sorta turned into resignation.
Oh, are those dragons showing up? Those are new. I'm pretty sure one of those alone could destroy this castle.
Sadly, I don't have any ideas good enough to actually make a real story. This was the best I could do with what came to mind.