I think you should be able to do all the 70 dragons. Your Elwin and Leon are a little stronger than mine I think. Bonding realms 65 are a little easier than 70 dragons so should also be doable.
Fire- easiest way is to do leongrisser it with 3 cheerleaders and a tank. Leon can stay out of the dragon’s debuff range, the tank sits right next to him to soak damage, and Liana/Tiaris/Sophia or Iris for support.
Ice - The toughest part is for your mages to survive the dragon’s physical attack. I had to put performer’s mask on Luna and some other tricks to debuff the dragon’s atk.
Thunder - This one I very narrowedly beat (literally on turn 15 counter attack), but your Elwin’s 6* and mine is 5* so you might have a easier time. I did switch Ledin to King from Templar so that he wouldn’t get one shot while tanking for Leon (this was back when I had Elwin as the one decent infantry/lancer. If your Bern/Sakura/Sumire/Vargas is built enough you might be able to attack the drake with more units, though that comes with their own complications with the mechanics)
Dark - it’s easy. Basically same as 65.