Karna who is 385 megatons, can damage Siegfried even though he has armor, Natsu who is 578 megatons, and has Dragon Slayer Advantage which should Damage Siegfried because his body is that of a Dragon's Skin, even with the Armor, not to mention, that Natsu is a human and isn't a character who has Ranks, therefore he wouldn't be affected by Rank Down, Natsu WILL be laying out Damage, not to mention he knows about Siegfrieds Weakness, but Siegfried doesn't know that he knows, which will catch him off guard, as well Natsu's Rage Power, and the ability to possibly devour Balmungs energy blast, Natsu has eaten Magic far above his tier before, so he should be able to do it here, as well Siegfried doesn't always start out with Balmung and it takes time for him to charge it as a servant, Natsu Takes this with extreme difficulty, but he does take it