Eru could easily kick everyone's collective butts. He made everything, and he can unmake everything.
Huan could beat Beorn. Even though Beorn could call other bears for help, Huan beat Draugluin and Carcharoth, not to mention Sauron in wolf form. He was the Wolfhound of the Valar for Eru's sake! He could
so kill a bunch of bears.
For Boromir against Imrahil I'd have to call it a tie.
Here's an interesting one: Gandalf vs. Gothmog (and I mean the Balrog, not the weak little tumorous Orc general.
)