made of sterner stuff

Made of sterner stuff - having a firm resolve; inflexible, unyielding:-
--Criticisms wash off me as water off a duck's bath. I'm made of sterner stuff.

This expression is part of a line from Shakespeare's Julius Caesar. In Act III Scene ii, Mark Antony, speaking at Caesar's funeral, answers the charge that he was an ambitious man: 'Did this in Caesar seem ambitious? When that the poor have cried, Caesar hath wept; Ambition should be made of sterner stuff'.:)

We actually use στόφα, which is of the same origin as stuff (Italian stoffa), e.g. Δεν είναι από τη στόφα των ηρώων.
So I would like to use «από γερή στόφα» here.

Or καλή στόφα.

Also «από γερή πάστα», not so common with «γερή» though, and a click lower than «στόφα» in register.

If it weren't for the Shakespearean line and such higher stuff, in some cases and lower registers I might use Δεν μασάω / δεν χαμπαριάζω [εγώ από τέτοια] in the first example for someone firmly resolved and unyielding.

And «φτιαγμένος/νη από ατσάλι».