Invading depends on what fog checkpoint you use your stone in. I've spawned literally right behind someone fighting the zweihander red eye knight in 1-3 and just backstabbed him when he almost got owned by it because he probably thought he had time to kill it before I showed up.
Only problem with doing that is you're more unlikely to find someone to invade in the middle of a map. Also depending on fast they're advancing through the zone, they're more likely to get to the boss fog faster which despawns you immediately. I've done that when I didn't wanna be bothered trying to fight a BP and risk losing my phantoms I brought specifically to fight a boss.
XI Wiki


