I will be delighted to start this discussion and answer all questions presented but they need relevant context - e.g. Cerbs are great for defense til you are hit with a sharp nuke and have no archers otherwise they are pretty useless. Assuming you are hit with a ranged nuke a good defend ratio 30:1 archers to Cerb behind a 25 wall with tower Phalanx and divine selection researched with Leo as hero.
Eryins for cross land attack, but again it depends what you are hitting doesn't it..i would always send 30 Cats with them too. or better send the Cats with a sea/ land nuke before if they are land stacking and relatively low on harbour.
Harpies for harbour checks..pegs to defend harpie nukes/horse nukes..its kinda hard without getting into boring math stats and probability plus with events chances are you are going to get rained on by Manti with has made sword pretty obsolete during these times.
I think due to their amazing lion flying powers we have to still go with manta per unit as best attackers but if you run into a lot of archers forget it..see its all relative to why you are hitting or being hit by..
the only thing i can say conclusively is Hydra are the best Myth units for ocean def and attack