Needless to say this should be left until you're done with everything else.
The best way to go about this is to get 2 level 4 fire dragons and play quest 20. Have your high level dragon use shield every turn and have the level 4s attack the boss. You can get about 70 perfects each fight this way.
An alternative is to fight enemies that also use earth shield, but the AI seems to suck about using it, so I find it's better to just keep doing quest 20. Water elements can sometimes heal themselves too, but again the AI is awful, so sometimes they simply don't. And sometimes they do it almost every turn. I don't see any pattern to the AI, so quest 20 is a lot more consistent for your energy spending.
Your secondary shield will need to shield about 2100 or more to fully block the damage. I have a level 27 Tribal dragon with level 4 shield that blocks a little more than 4k, to give you an idea of what you're looking at. You should presumably have a dragon that can fit this role by the time you finish the other achievements.