...she can put Kirby down for good is via summoning, which requires her to transport them into the dream world...
They're already in the Dream World, but she could simply BFR Kirby, summon people, and then come prepared for when he comes.
While each one happens really quickly, nothing's stopping Kirby from teleporting over to her and possessing her.
Except BFR. An instantaneous, Multi-Universal range BFR.
Nothing states that Yuyuko's dearth manipulation is absolute, and you can't resurrect. Not to mention anyone she summons that can hax gets one-shot, and dying is only one method for Kirby to achieve Ghost
Yuyuko's death manipulation is soul based, and Doremy doesn't have to summon Yuyuko. There is Yukari, who has a far more potent hax, and Kaguya, who can stop an opponent in a literal instant.
A single Star Rod regulates the dreams of everyone on a star sized planet, so I'd say it would give her a run for her money. Plus, Star God's been shown to get Kirby out of situations just like this before.
Please read what Saikou said. The Dream World is also more of a physical realm than a literal dream.
Already taking place in the Dream World gives an unfair advantage and that's against the rules.
See: The previous statements on the Dream World made by Saikou and I.
Kirby can wake up from sleep, as he has a technique for quickly waking up from detrimental sleep. Heck, putting him to sleep heals him.
But does it work against Doremy's method of force-sleeping? To quote, "Kirby's dreams are only of the mental variety. Meanwhile, in Touhou, it's more like putting you to sleep and shoving your soul into another world."
There's still the very real possibility of Kirby copying Doremy and anyone she summons.
And dying before he can do that is also a real possibility. Not only that, but Kirby's power mimicry is not really that great. For example, if he swallows Reisen, he would gain lasers, but not the wave manipulating, insanity inducing gaze of Reisen. Rather, he gets his own version when he copies an ability, rather than their powers in full.