If you're not using high end resume key word screening applications, write a job ad that requires a cover letter. Require in the cover letter candidates answer a question or two posed in the job ad. Eliminate all candidate who 1) don't send a cover letter and/or 2) send a boiler plate letter that does not answer the questions posed in the ad.
Other easy screening criteria 1) spelling, syntax or grammar errors in resumes and cover letters 2) boiler plate objectives (for candidate that still use them) that don't fit your company or the position you're presenting. 3) Experience not consistent with the position you have; for example, candidates with several years of experience submitting resumes and cover letters for entry level positions
"What is the hardest obstacle in the process of posting job openings?"
Logging into and posting ads on all the different sourcing places.
