Some of the things i learned from the game is never fight fair.
Use terrain, high ground is awesome. Crowd control spells are great. Concentrate your fire, it is better to do 1 hit point of damage if it kills an enemy, then 12 hp of damage and it doesn't kill an enemy. The fewer "guns" firing back at you the better, and an enemy with 1 hp still hits as hard as an enemy at full hp. using terrain to block line of sight from enemies forcing them to come to you and often enemies are spread out so they will rush towards you together but arrive in a staggered fashion. This means you can concentrate fire on a few enemies with a few attacks in return. Also you ranged party members can move out from cover attack and then move back into cover to get a few attacks in on approaching enemies. If you run into melee centric enemies attack with range than run away, this strings the enemy along getting more attacks in before they close the distance. Remember that shove is a bonus action so pushing people off ledges can give you tactical advantages without costing you your turn.
Grease + firebolt is a great combo that can turn the tide of battle. Do not underestimate it.
