Having played enough games to get 202 trophies at this point, I think the strongest build is a combination of “triggers continuously” items that produces Rush or Slow, together with the sword that applies Poison and charges on Rush/Slow. That has the net effect of stunlocking the opponent and poisoning them.
It’s essentially invulnerable except for an opponent that also has infinite-stunlock items. Against such an opponent, the winner will be determined by some combination of the king hurrying the battle up, who has the most max health, and who got a few lucky shots out before the mutual stunlock.