Most humanoid enemies are pretty vulnerable to the Daiba-Washi spirit skill on Way of the Samurai (if you have the Guardian Spirit Talisman in the Omnyo trees use that; otherwise, use the Moment Talisman loot items). It'll blast people up in the air and let you get a free ground stab or combo. Lets you cheese through fights that are otherwise more challenging.
Fox boss is a bit trickier. Typically level doesn't matter as much as gear/build in Nioh, but being under-leveled against the fox is rough due to the health pool and damage output. If you have a Guardian Spirit with a long living weapon uptime (ideally earth), that can also help in tandem with using amrita stones.
I am using the magoichi saika build with ulimited ammo 0ver 50% and i have the hanzo and hayabusa sets and that other red ninja. And the red deamon for spear use if i feel like it.