Get the Whitepaper called A Hands-On Approach To Linux Privilege Escalation. This document is intended to provide multiple techniques that a pentester can use to escalate their privileges and gain access to higher roles.
You can download it from the following link: https://packetstormsecurity.com/files/download/160908/handson-linuxprivesc.pdf
Source: https://packetstormsecurity.com/files/160908/A-Hands-On-Approach-To-Linux-Privilege-Escalation.html