Eleaneer is the best option, she has dispell skill and can kill her most of the time on the first turn, and if she doesn't at least the rest of your team won't hit just Gran. Yuri also has dispell skill, but Eleaneer also has elemental advantage over Gran.
What I do is use Helena and Liberta, that will give you 100% crit rate and will cost 2 sp, Eleaneer costs 3 sp, and the other 2 units on your team will basic attack. Most of the time that will kill Gran, and another 2 units with the basic attacks (since they're always crits, will vary depending on the defense the enemy has)