The boss will use a turn dispelling the reflect, but its really not as easy as that yeah. He'll spend a turn on that, but unless he's also slowed, he's very likely to get a turn and blast your party before you can get a second reflect up. It basically saves you from x number of his attacks, so you want to save them until the tail end of the fight when he starts casting Flare Star and Southern Cross, which do massive damage to all party members.
Of course, you also need Def because the dragon does single target physical attacks that hit hard. Blind Buster does the trick, but getting it AND Venom Buster to land can be... eh. If I have to choose between the two, Poison is a lot more crucial than Blind.
The fight is a balance between killing it quickly and not completely blowing all your abilities at the start, because the dragon gets more lethal the lower his HP is, so you want to be able to blitz him once his HP is at a certain point.
