大学出たての新人は資料を作ったり、コピーをとったりと裏方仕事に徹するのだろうと、勝手に想像していたのですが見事に裏切られました。

、、。

I'd imagined — without anyone saying so — that fresh graduates would stick to behind-the-scenes work like preparing documents and making photocopies. I was gloriously wrong.

Literal

University just-graduated newcomer [topic-は] documents [obj-を] make-[one-thing-たり], photocopy [obj-を] take-[one-thing-たり] [and-と] back-stage work [to-に] devote probably [quotative-と], arbitrarily was-imagining [but-のですが], splendidly was-betrayed.

大学出たて ('just out of college') uses ~たて suffix meaning 'just (done) X / freshly X' — パン焼きたて ('freshly baked bread'), 出来たて ('just made'). The ~たり~たり pattern lists example activities non-exhaustively. 裏方仕事 ('behind-the-scenes work') is a compact compound. ~に徹する ('to devote oneself fully to') is a formal collocation. 勝手に ('on one's own / without authorization') plus 想像していた ('had been imagining') conveys an assumption made without input. 見事に裏切られる ('to be splendidly betrayed') is a fixed ironic phrase for 'expectations spectacularly defied.'