My school employs undergradates graders for most of the undergraduate math courses, and this year I have been tasked with grading an intro to proof-based linear algebra course. This is students' second exposure to proofs after the $\epsilon$-$\delta$ intro to analysis sequence, so unfortunately they don't know how to do anything but fill the blanks in an $\epsilon$-$\delta$ template, i.e. they haven't thought through a logical proof really. This makes grading a bit tough since there are around 30 students writing horribly long, incorrect proofs for what could be 2 line proofs.
My question is, how can I grade the homework assignments more efficiently? I've tried the 'one problem at a time' method where I grade one problem on every single sheet and repeat, and I've tried the 'one sheet at a time' where I go linearly and grade every single problem on a single set and do this for each student (looks like Fubini's theorem applies here :p) I'm interested in other graders' routines.