Solomon is weaker than Goetia even as equivalents of "Grands" due to Goetia basically being the first Servant.
It doesn't mean Solomon is trash, it's just Goetia is above him in respects to their abilities.
Of course, any one Grand can solo Slime unless you go with whatever the hell Rimuru becomes at the end...
I already talked about how retarded the first part is(And how insane the NLF is towards SnS' New World Order is when if it was that good, why didn't she do it against Shigaraki? She wouldn't know he's Heracles considering Greek shit would have long since vanished with the shit that happened in MHA and he would be called Berserker at that(and even then, considering how widespread Quirks and the changes to a person are, she would believe he's All-Might's Clone first before thinking he's Heracles)
The first part has been consistently thrown around despite the fact that:
1. Shirou only talked about the explosive force of Rin's Gems, never how much power they actually have(in the same fashion Archer's Base Arrows can destroy a House =/= how much power they actually have)
2. Her typical Gems explicitly can pierce Heracles which even if you bypass his God Hand never guarantees you can seriously injure him to begin with.
So yeah, SB has this thing to where they take things at face value whenever it benefits them... which is usually when it's to downplay something(Or in the cases like MHA, straight up REMOVE context so that Deku's Danger Sense seems more broken than it actually is)