That’s up for debate. While she’s portrayed as the biggest psionic powerhouse, I don’t think that means she’s invincible or immune to psi blade damage. Blizzard gave her a bunch of in-game cocoon voices and a cocoon model because she can be killed even by regular units (and then revived). Warfield has his own dialog for when his troops kill Kerrigan.
Zeratul lobbed off one of her wing blades. His arm got hurt, sure, but it’s not impossible for him to have timed his attack better and cut her head off instead.
Now Alarak probably doesn’t have Zeratul’s stealth abilities, but he’s probably got way more psychic power from the terrazine and years of using supplicants. That’s why he dominated vs. Vorazun. If he’s sneaky enough, he could get Kerrigan as well.
I’d put the odds at 70% chance of Kerrigan winning. Obviously Kerrigan is stronger and way more durable, but I think Darth Alarak has a chance.