I have done that on many servers, maybe not so many Mantis, but Griffs were designed to mess up mantis amongst other things they are useful for... I use Griffs more than mantis because they are very good against the standard slinger defense.... Flat out, per favor point, Griffins are better units against what most people turtle with. Plus, if you have archers defending against hops, those griffs are gonna tear them up... I love sending 20 griffs against 600 archers and whatever else they have around...
Swords sort of defend against Griffs, but then you have archers defending against hops, unless you have a Hoplite Nuke, you fail to defend against Griffs on the fly... Easier to build vs Mantis since it requires less favor...