R.Ost is among the most well-equipped pushers in the game. He has fast movement speed, high health, enough range to hide behind most peons, and a rapid Area Attack that will shred your cats. The icing on the cake is that he can do Critical Hits, which make fighting him much more inconsistent. Fortunately, he has five knockbacks to reduce his heavy pushing power.
Long-ranged attackers such as Crazed Bahamut Cat, (Crazed) Dragon Cat, Paris Cat and/or Drama Cats will do fine against R.Ost in earlier stages. However, in much later encounters, you may need the use of more crowd-control against R.Ost. Tourist Cat and/or Glass Cat can be good at this, as long as using crowd-control is not exploitable by the other enemies in the stage. Another, less applicable strategy for R.Ost is to use Awakened Bahamut Cat. When used correctly, he can chip off an R.Ost by being timed to strike during R.Ost's backswing animation (the second peck), then let the R.Ost be finished by the rest of your attackers if Awakened Bahamut Cat did not kill said R.Ost. For this to work, Awakened Bahamut Cat must be spawned while R.Ost is frozen or during R.Ost's backswing and there can't be any other enemies in front of R.Ost. Freeze effects, such as the Thunderbolt Cannon and Tourist Cat, can make this easier to time.
A Metal enemy that only appears in the Golden Week and Silver Week events. It has the usual Metal ability of only taking 1 damage except for Critical Hits, but it has much lower stats all across the board and cannot perform Critical Hits itself.
R.Ost's full name is Rich Ost, which combines to give you Ostrich.
The joke with his Japanese name and description is that he belongs to an ostrich club whose members are all named Ostrich Club; it's also a reference to the Dachō Club, a Japanese comedy trio.
If you pay attention to R.Ost's attack animation, you can see that one peck does all the damage, while the other does not deal any damage at all. This allows you to time units easier in stages such as Realm of Carnage.