Servent suggestions for defeat Siegfried

Best way is to get Kintoki (Berserker) with upgraded Princess CE equipped (probably take some time before you find and befriend someone with him tho). He mops floor with Siegfried as his NP ignores defense (in this encounter it ignores half of it, as far as i can remember). Also, Mata Hari with lowering defense debuff and Charm works wonders as support. Other than that... well, get your taunts (as your Mash still 30th level, i think you ain't got taunt on her) and damage buffs ready.

tl:dr
Bring something like:
Kintoki (Berserker (with Princess CE)) - Mata Hari - Leonidas \ Georgios (best if equipped with something like 500-year Obsession) -
As Mystic Code - basic Chaldea Uniform both boost damage and provides protection against NP in first Siegfried turn.

You need strong supp with huge Def buff and strong heals. Burn/curse/poison team need a lot of backup. As i can see your team comps lacking in solid supp to do that also your Mashu still underleveled.

Like mentioned above, go for Dps def ignoring router. You can clear it faster with just Emiya alone or wreck Siegfried with Kintoki Golden spark.

You would want to build a team around Ignore Def Servants like Emiya, Kintoki or even Lu Bu. Bring supports like Mash and another tank like Georgios or absorb the hits for your main attacker. If you have any healers like Hans it would add sustainability for your team.

I ran a team with Lu Bu + friend support Emiya both with MLB Princess of the White Rose supported with Mash, Georgios and Hans for tanking and heals with Chloe for a last stand.

Hope this helps!