Evolving as a Pseudo-True Dragon just further cement that fact. Great Demon Lord is just a title hold by an Awakened Demon Lord level being who have many subordinates sitting on the level of an awakened Demon Lord under his reign, which is a qualification he already fulfilled even before eating Veldora's body. The slime ends up beating the Parasite, 7/10, and clears rather solidly. The Slime trap has raw power and durability close to Arthur's, as well as some sickeningly broken hax and the ability to boost his reactions by millions of times. Round 3: Rimuru clears, only getting a moderate fight out of Rou and Arthur, though he would have a far more drawn out fight with the latter. Pair that with monstrous endurance and high level regeneration, in combination with his Void and Dark Magic hax, Arthur has a good chance of giving the Slime a bad day. Arthur was casually overpowering monsters who could threaten large planets and shrugging off everything they threw at him with ease, including high level illusions, soul attacks, mind rape and ludicrously hot toxic fire. I'd say he gets past Rou after a good fight, then stops at Arthur, as Rimuru is once again outdone in terms of sheer DC and durability, when it comes to the Parasite. his DC/potency is much higher here, same with his speed and hax, so Rou has much less of an advantage compared to the first round.

The sheer stat difference is just too huge, even Rimuru's big arsenal won't help much, considering that Rou is pretty versatile in his own right.

Demon lord true rimuru tempest