Even with preparation time, Doctor Doom would be at a loss against Sise-Neg. For one thing, his magic power would be immediately drained. Secondly, that prep time could not help him against someone infinitely more powerful and with much wider capabilities at his disposal.
The instance of Beyonder getting his power drained by Doctor Doom with prep time is often brought up, but it is usually not understood correctly. Beyonder was defeated by a prepped Doctor Doom in a reality where Beyonder himself subconsciously gave everyone anything they desired. Additionally, the concept of Death itself was skewed so that all the inhabitants in that reality were particularly hard to kill.
Doctor Doom in that reality wanted Beyonder's power more than the Beyonder wanted his own power, . There is also the plot that stipulated that Beyonder had to be limited - lose his power - so that he may become curious about the feeling of desire - wanting his power returned to him - in his weakened state, which paved the way for Beyonder's visitation in Secret Wars II, which was a quest for Beyonder to understand desire.
So... Beyonder gave everybody the power to make their desires turn to reality. Even when Beyonder returned back to his Beyond realm, his leftover power was still turning everyone's desires into reality.
Here's an example of Captain America controlling atoms just by his will alone due to Beyonder.
So the reason Doom was able to take Beyonder's power was simply because he wanted them, meaning anyone that wanted Beyonder's powers could have taken them. We even see Captain America turning into a manipulator of matter all just because he wanted his shield back.