The issue with Griffith saying he's a master tactician is this: his whole strategy usually revolves around sticking Guts in the middle of a few hundred dudes and saying "have fun bye". It doesn't even improve later on, because his troops are monsters - they just wouldn't lose regardless of what he told them to do.
Griffith is apparently immortal, can control gravity, and has a great big apostle fanclub. He warps reality with the astral plane. It isn't even a contest.