Could Expelliarmus beat Avada Kedavra?
Avada Kedavra is considered unblockable; as seen in the books, not even a strong shield charm like Protego Maxima can protect against its killing power.
Harry was known to use Expelliarmus to fight Avada Kedavra to a standstill, anyway – although this is mostly because Harry and Voldemort’s wands were twinned, and Harry’s lack of fear made him stronger than Voldemort during the duel. Against any other wizard, Expelliarmus would fail against the Killing Curse.
What are the limits of the Accio spell?
According to Wonderbook: Book of Spells, an official Harry Potter video game, Accio comes with a list of rules and limits.
For starters, an object needs to be pictured out specifically in your head to be summoned. However, this rule has been ignored multiple times throughout the series. Secondly, most living things can’t be summoned, but food can. Third, objects can be protected from Accio with anti-theft charms, which are commonplace in wizarding shops.
Lastly, Accio normally needs line of sight to the object to work. Then again, Hermione, being the prodigy she is, found out that you can summon objects as long as you knew their location and appearance.