Uhm... Leona only had +25 MR with items. Her shield would give +65, and Galio's would have given +90. So she has about 258.4 MR at level 18 with all those buffs. 258.4 is a lot, but it's not as much as she should have had. (She has no MR from Masteries or Runes). She never got to 18 so her MR would be lower still.
Though this is excluding any bonus MR Galio's shield would have given Leona's shield (scaling off Bonus MR), if it would even affect it at all.
What I'm trying to say is that seeing your build, and thinking it was well before the end of the game, it's not all that surprising since the Leona didn't really build to combat you at all. Which was pretty dumb of her.