I am going by the Wikia-Pages here. Thought, yeah, I also had to notice Concept Manipulation is not noted on Servant Pages. Why, I actually do not know. F/SN clearly states that NP are Crystalized Ideas/Concepts. I mean, there are servants that do not even exist in History but are instead manifestations of that Story.
If Luffy can hurt Mordred does not matter. What matters if he can hurt Semiramis. I'd use Statistiks for Semiramis, but Mordred's are way to varied to do that to crossscale and Semiramis Nature makes it rather hard.
What am I supposed to say? Sabers have extreme magic resistance and Semiramis is still able to hurt one. If I were to go by Nasu-Statistics, Semiramis is somewhere between 7A and 6B in her Throne-Room for her EX-Magic Rank. Normal A is mountain Level, EX is immeasurable. But I cannot use these statistiks since they are to varying and undecided.
Basmu is noted to be a "Surekill" NP-Beast, but Mordred was able to cheat her way through it because of her MR and Semiramis being to rushed to use it fully since she had no time to summon it completely on the fly.
But screw it. I am going to clearly write out what I think now. Since I myself have been kinda spongy with my Arguments here. I am sorry. That was not my intention.
First of all, the huge Elephant in the Room: Semiramis' poison. Problem is, none knows how strong it is, compared to Magellan. Judging by comparing effects, she should be around there somewhere, maybe below, maybe above. Never poisoned Luffy was able to fight poisoned for a while, but massively impaired, while Mordred dropped after seconds. It seems tied to her Magic Skill, which would imply Mordred did not instantly die because of her MR. So I assume, in term of this fight, that Luffys Instantaneous State in the Room will be below Mordred's state of Damage, but still impairment. I here, take sluggish movement, imprecision and other simmilar effects. When she activates her NP, all her Spells gain the same Level of Poison, which she can also change to every other type of poison on the fly. This seems to include every type of mythological poison, too, considering that she can faithfully replicate Hydra poison on the fly without any mythological ties to Hydras.
I'll leave out that Semiramis NP is noted to be a Sure-Kill NP, since that seems rather Outlier in htis context.
Next: Basmu. Not much I can say there. Basmu is supposed to be stupid deadly. And powerful. The Basmu she summons here, since she is allowed to Summon it beforehand, will be far stronger than the one in F/Apo, since she is not rushed this time and can summon the Full Beast. Problem here: Divine Beasts range from Mountain to Country to Planet Level. So, how powerful is it? It is just noted to be so poisonous you die. There are Dozens of them, all with varying powers and Resistances. Only true Scaling seems to be Tiamat here, since Basmu is a child of her. But Tiamat is 5A, which I rather doubt Basmu is. So I'd put True summoned Basmu somewhere between High 7C and 6C, to make it kinda fair.
I'd however doubt that Conquerors Haki will work on it, since it seems to not work on people close or equal to your own power.
Magic: No clue how to write that up. She can harm people with B Magic Resist. How good is that? Well, as far as I know, it's "nullify anything below 3 verses and weaken anything above". Eh. Does not really help to quantify how much stronger her Magic would be on Luffy instead of on Mordred. Considering however, that they did mentionable damage to Mordred, I'd say they will do mentionable damage to Luffy and again, unlike her, he has no Armor or MR to shield him from the Damage.
Her chains are extremely fast, able to blitz Mordred. Luffy is faster, at least in Gear 2, than Mordred, so I would say they do not Blitz, but should be capable of at least chasing and harming.
She can also generate thousands of Minions with her Dragon Fang Warriors.
Lastly, her Barriers. They have shown to Block Clarent Blood Arthur, which is above 7A and as it seems, Rider's Pegasus, which is solid 7A. So I'd reason they can block Luffys Gear attacks.
And that pretty much summs it up for me. I just do not think Luffy will win while impaired with poison. Without it, it would be a whole other buisness. But his impairment will only get worse the longer the fight lasts and Semiramis will have a incresingly easier time to hit him.
Again, sorry if I was a jerk about my arguments before.