M. Ost is a very formidable foe, even in comparison to the common R. Ost. He has high health and damage, extremely fast movement speed and attacks, decent range and guaranteed knockback + slow + Curse, all of which allows him to wreak havoc upon your units if not dealt with properly.
He does have a weakness, however, that being a whopping 10 knockbacks. Therefore, if your cats can do enough DPS to M. Ost, he will be knocked back continuously until death, preventing him from causing too much damage.
Although M. Ost appears on paper to be R. Ost on steroids, it really isn't as terrifying as it may seem. Its combination of 100% knockback and slow abilities can actually help push your heavy ranged attackers back into safety when they're hit. Once you have a sizeable stack of mid-ranged attackers, you should be able to trigger its knockback before it destroys your stack. Its slow ability also somewhat alleviates the suicidal tendency of fast ranged attackers, such as Super Galaxy Cosmo, Fire Squad Kachiyama and Saber.
When fighting this enemy, you can use a fairly similar strategy to what you would use against a regular R. Ost. If you have any reasonably strong cats that outrange M. Ost, like the Dragonvariants, they should be able to hold him back.
Cyberpunk Cat also works very well against him. If the stage gives you the option to stack long-ranged cats, that can also be used to keep M. Ost constantly knocked back. If you've finished the painful process of completing all 4 stars in SoL, and have completed the Eldritch Forces Invasion, Idi:N is a fantastic counter to M. Ost, due to his massive damage and short attack animation. However, timing is an absolute must for rushers, including Idi:N.