You must use at least five copies of two cards in the selected group.
Does it means that you have to use 5 copies of the 2 cards selected in the group or the sum of the 2 cards must be equal to 5?
Can you select more than 2 cards of the group?
e.g.
3 flooding, 2 mirror shield is ok?
2 flooding, 2 mirror shield, 1 poison is ok?