"Sudar tinta" = To sweat ink*

To sweat blood / buckets. People also say "sudar la gota gorda". Much more informal is the quite common and colloqial "sudar como un cerdo", just like the english to sweat like a pig.

"Estoy sudando tinta con este calor" "Qué cansado estoy después de tanto subir y bajar. Estoy sudando tinta"