That is so ridiculous though that it can't be a feat for Oscar. There is simply no way he could beat something that could beat a level 2 as a regular human without preparation, so the Wepwapet simply can't be that strong normally. And I'm not sure being on steroids helped Michael that much in his fight against the lightning monster, since he didn't get physical with it.
Anyone got the stat sheets for the Wepwapet and the lightning monster? Comparing their stats would make this much easier to conclude. If I remember correctly the lightning monster is like C class and Wepwapet is E or D class.
The Wepwawet never got a monster report, but you are right in that it'd probably be a D rank monster, maybe D+ since it's explicitly stated to not be that powerful by a Level 2 MC, just in possession of an extremely dangerous power with a massive range that can kick in before you even see it. Dangerous enough that if you try and kill one with a mantis punch, the Level 2 MC can get slapped across a cavern, accelerated faster than a bullet into a wall and be near helpless whilst having his mangled body eaten alive faster than it can regenerate under the light's effects. It legitimately almost did him like Valravn did in Dead End 24, turned into a renewable food source.
There are some key differences between monsterified Oscar's Red Light and a Wepwawet's Red Light. Oscar's brought its target to a complete stop, but had a cooldown and could be broken out of with raw strength. On the other hand, a Wepwawet's version has no cooldown, but slows things to a crawl instead of actually stopping them and actually takes some time to kick in, though it offsets this with a large range and getting more effective the longer a target's in the light. That difference is actually what makes it killable, because at close range it actually can't stop attacks going at supersonic speeds. Oscar's an ex-soldier, and gun laws in Sector 1 are so lax the only thing off-limits to civilians is explosives. It certainly wouldn't have been easy for Oscar, considering that it's still a car-sized monster with light-based powers and inhuman strength, but at his reported skill level, smart use of the environment and Wepwawets being glass cannons make it actually be possible. Oscar took it out ironically the same way he ended up getting taken out, going for the eyes.
Also, the formula A-8163 absolutely helped Michael during that fight against the Leigong, he almost got disembowelled mid-fight. In fact, he got himself killed fighting a monster just like Oscar, because he injected himself with so much magical trenbolone his ass was about to die 15 minutes after the fight because his body couldn't handle literally mutating beyond all human boundaries. For all intents and purposes, he had to pop so much drugs he was operating at the level of a low level superhuman to win.