Should I use in before managing?
I showed great multitasking skills in managing a heavily trafficked office while still finishing on time.
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Assuming this is for a resume, "in managing" is better.
With and without "in" are grammatical, and have different meanings.
With "in", it means "managing" is how you showed great multitasking skills.
Without "in", the reader will understand "while" instead, so it means you showed great skills while managing, but you're not saying how you showed those skills. Saying how you showed your skills is essential in a resume.