[The Core Dynamic: Paranoid Slime vs Showman Demon]
the battle between rimuru ( light novel rimuru of course) and Anos is like ' a paranoid slime ( who views no one as weak and is always extremely careful) ' vs ' showman demon king (who believes he is above everything and everyone). ' the battle will be heavily dependent on when will Anos realize Ciel's analyzing him from head to toe, since We know Anos loves to toy with his enemies and let them act first, famously asking, "Did you really think killing me would be enough to make me die?" if Anos doesn't realize the gravity of the situation ciel will have all the time to map Anos and how his Source Destruction works, However, if he realizes the gravity of the situation before Ciel can map him out, the battle will go the other way around.
[First Scenario: Rimuru's Stealth Strategy & Ciel's Trap]
After mapping out Anos and how his Source Destruction works, Ciel will either synthesize a perfect conceptual barrier to nullify it or use Void God Azathoth to swallow and isolate Anos's magical outputs into Rimuru's imaginary space, since Anos is arrogant and trying to prove a point, Rimuru will be constantly throwing flashy, varied attacks that Anos can easily flex against and destroy, Rimuru will be FEEDING Anos's desire for showmanship!! While Anos is busy looking cool and standing his ground to prove a point, Ciel is silently dissecting his entire essence from head to toe. And By the time Anos realizes the attacks were just a distraction, Ciel will have already rewritten the rules of the local continuum to entirely nullify his magic source! Or Rimuru can use Supreme Nuke: Maiya, which Rimuru can theoretically use his time-transcendence to isolate the entire timeline they are fighting in, leave it behind as a dead end, and simply re-create a brand new version of existence where the concept of Anos Voldigoad never existed in the first place... Rimuru is a natural-born actor and a bit of a panic-merchant, but let's not forget Anos's Silver Sea (where a world deeper in the sea treats shallower worlds like mere paper/projections) No matter how deep a world or source is, if it manifests an action or magic output in the fight, that output contains readable data. Ciel analyzes the 'code' behind the action while Anos is distracted. Silver Sea scaling only matters if Anos instantly treats Rimuru as a deeper-world entity that requires immediate, full-power destruction. But in-character, Anos evaluates his opponent first. The moment he holds back to see what Rimuru can do, he hands Ciel the time window needed to process his magic parameters. In short: Rimuru's paranoia is his biggest advantage to feed Anos's ego and completely map him out.
[ Second Scenario : Anos's Win Condition - Venuzdonoa & Logic Erasure]
While Anos is arrogant he isn't stupid, because we all know he has thousands of years of combat experience, if Anos's tactical instincts got trigger he will immediately realize that ciel is analyzing him then he will unleash a conceptual killer like Venuzdonoa to delete Rimuru's source before the analysis hits 100%, he can nullify the very concept of Ciel's calculations! Venuzdonoa makes it so that no matter HOW smart or fast a protective barrier is, the "reason" or logic for that barrier existing is deleted If Anos uses this immediately without talking or flexing, Rimuru's defenses vanish INSTANTLY, let's not forget his Magic Eyes of Destruction too (or Magic Eyes of Chaotic Destruction) although yes he CAN perceive the fundamental structure of all magic and intention instantly, so he’d see right through Rimuru's flashy distraction attacks before Ciel reaches 100% analysis! But the problem is that he doesn't have his eyes always active according to the lore. He actively suppresses them! Since his eyes are so absurdly destructive that merely opening them causes the world/universe around him to break down and collapse, he actively keeps them suppressed into the weaker Magic Eyes of Destruction (or Magic Eyes dyed in Destructive Purple) and keeps them turned off or at low output during normal interactions or casual fights. Because Anos doesn't walk around with his eyes flared up unless he considers an opponent an immediate high-level threat, he wouldn't have them active while playing along with Rimuru's "flashy distraction attacks." In other words, Anos's ego is his biggest weakness in this scenario.
[ Short Summary]
Rimuru Wins IF: Anos doesn't see Rimuru as an immediate threat quick enough and decides to play with him, giving Ciel an open window to analyze him from head to toe.
Anos Wins IF: Rimuru makes one mistake and alerts Anos, forcing him to use Silver Sea Ship / Venuzdonoa to nullify the very concept of Ciel's calculations before they reach 100%.
The battle is heavily dependent on when Anos realizes how much of a threat Rimuru is, how long Rimuru can feed Anos's ego, and how long Ciel's calculations take to map out Anos.