Assembly Language is rigorous to learn. It is difficult to write complex programs in assembly language but, to become a successful pentester, Assembly Language is important to learn.
Learn this language not to build the code but to break the code!
Often times, Assembly Language is used to find the 0-day vulnerabilities because debug will only happen at the assembly level. Additionally, this is the best programming language for pentesters to perform Malware Analysis, Reverse Code Engineering, Software Vulnerability Analysis, and Bug Hunting.
You must know basics of C, C++ and Arithmetic logic and some knowledge of Digital Logic & Design, else it will be extremely difficult (Even most experience programmers struggle with it)....
Resources:
Assembly language Seventh Edition by Kip R Irvine
PS, you will need alot of other tutorials/guides to get grip of it if getting stuck....
Some Assembly guys on defcon etc, presentations: