Boros Reckoner and Priority

[The1337Magician]

So I attack with {Battering Krasis} that is a 5/4 and a {Druid's Familiar} that is a 4/4 and a bunch of other creatures. The opponent only has {Boros Reckoner} to block. He says he blocks the {Druid's Familiar} to deal 4 damage to {Battering Krasis} before it deals lethal damage. I said all damage is dealt simultaneously so the trigger wouldn't go off till after all combat damage is dealt.
Who is right?

[Kagain123]

You are, unless your creature has first strike.

[Paraluke]

You are absolutely right.