"Of all the Heroic Spirits and Masters present, perhaps only the Conqueror and Weber's group posed the least threat. My power was too heavily restricted in this world. If the other Servants truly joined forces against me, even I would not be able to withstand them. Gilgamesh alone was enough to keep me occupied." Meruem might have made fighting two opponents at once look easy before, but in truth, he had only taken advantage of Diarmuid being forced to attack by a Command Seal. If Diarmuid had joined forces with Qing Ming at full strength, Meruem would have had to pay a price to win.
And just because Meruem's Another Dimension Space completely countered Gilgamesh's Gate of Babylon did not mean one of Gilgamesh's strongest abilities had been rendered useless, or that Gilgamesh was no longer dangerous. Anyone who thought that was simply being far too naive. Without protagonist plot armor, Gilgamesh's title as one of the strongest Heroic Spirits was no empty boast when he got serious.
When Gilgamesh unleashed Gate of Babylon at full power, he could fire at least three thousand Noble Phantasms in a single volley. That move alone was enough to eliminate more than ninety percent of all Heroic Spirits.
Which raised the question: could Meruem's Another Dimension Space swallow a simultaneous barrage of that many Noble Phantasms? Earlier, because of Tokiomi, there had been no confrontation between Gate of Babylon and Another Dimension Space. Even Meruem himself doubted whether he could devour so many Noble Phantasms in one go.
This world's restrictions on him were simply too severe. Even the golden combat techniques in Meruem's hands had been limited. Otherwise, even if Gilgamesh himself appeared before Meruem, he would only be another dish on the table.
And one must not forget that Gilgamesh had