I try and look at nature and how animals attack and place monsters into a bunch of groups. Things like: do they actively hunt the "races", are they stalkers, ambushers, territorial, pack hunters, opportunity hunters, scavengers, just mean, mindless, etc. Once I have that I will build my tactics.
Example: goblins in my game are pack opportunity territorial in my games, that means they will attack in their lands, when they have a force greater than the one they are about to fight, and will take any opportunity to weaken that force; like driving another monster into the party or trying to weaken it from a distance but they will not engage as long as they think the party is more powerful.
Example: Displacer Beast is an active stalker & opportunity hunter, they will pick a target out of a group and focus on it waiting to attack. they will only stop hunting IF the creature escapes from an attack.
Example: Owl Bears are territorial and just mean; you enter their territory they will attack.