"As hard as nails"
Translation:Mgumu kama msumali
I think they spell it with "r" earlier in the course and I think that's the standard advocated by BaKiTa, the Tanzanian authority on Swahili.
As for the question about whether it's plural ... I'm not sure. If this is something Swahili speakers say idiomatically, it should be left, because maybe they literally say "As hard as a nail" in the same contexts as English speakers say "as hard as nails", but if it's just a direct translation and not an idiomatic expression, then yeah, it should be misumari.