All Eromon you encounter (except the rare) are Ruby element, because Karen's have high def, they usually take 8 hits of 10 dmg or 1 hit of more than 74 dmg to take out.
If you have Sapphire Eromon with ~2200-2400 atk you can take them out easier/faster
The rare for Area 7 is Belial.
So if you have extra Tomes to give your strongest Sapphire Eromon specials that have more dmg to ruby or more atk that will help make Area 7 even easier than it it is.
Also ruby resistance will make your sapphire Eromon take very little dmg in Area 7.
It's nice because the next areas 8 and 9 also are all single color Eromon (except in case of rares).
Area 8 = all green/Emerald, Area 9 = all yellow/Topaz
