Looking for some combat advice

[Fashionisdanger]

Glad to have helped!
About synergies, I have found very effective for example having a drow or a mage cast Darkness, hide and then unleashing spiders (who have blindsense), or a pack of kobolds trowing oil all around while one barring a door and another putting the whole room on fire... Enemies may also have allies shooting arrows from wall slits too high to be reached, or prepare actions to be unleashed when an ally goes into melee range.

5e monsters seem to be designed to work either as supports or the main treat of an encounter, and this make the most of them viable even at higher levels, regardless their CR.

This website helped me tremendously understanding some monster potential
http://themonstersknow.com/

Of course, sometimes is fun having dumb monsters that helplessy fall into players ambushes, but you can get really creative with your encounters with a little of research.
Also, roll initiative once for each monster type! So you can rush them with more enemies at once (it is also intended to work this way RAW).