To be honest the best "Trick" I can give someone for leveling is that killing mobs 1-3 levels below you is always better than 1-3 levels above you. Experience per level up or down isn't a huge deal especially when grinding speed is important to you. Pulling 3-4 mobs 2 levels below you vs maybe accidentally pulling a couple mobs a level ahead can be a very clear way to view ease vs difficulty.
For me I highly recommend you kill your way to next quest / zone you're headed too. Always pick up everything, and try your best to really always be doing something. The biggest way to get ahead in this game is going to simply expanding overall kill total.
You really need to think about normalizing your level in your head to adjust how you can go faster. Completing quests 5 levels below you isn't going to be dropping XP off while completing red/orange quests are just going to increase XP slightly. You need to be grinding mobs heavy enough that your level isn't surpassing your quest log. If you have quests you're getting close to out level those should be priority so you can benefit from mobs / xp there without passing it. If you have orange / red quests you almost never want to be completing them right away unless its an instance you're knocking out with the bros.
In terms of killing mobs your level something to think about is you need to kill roughly 16,238 mobs give or take a few at your current level to hit 60. Anytime you're cutting into that number the better. The experience per hour you gain from pure quest to quest vs killing isn't much lower / higher as you progress. The more quests you have to do at your current level vs being stagnant the better. If you rush everything without grinding between zones you'll have some level's in 30's 40's 50's you have to purely grind out to be able to reach next quest zone.