Quote from Jegpeg >>

Something you can potentially do as a caster is cast APE spells above the target.  If you eyes are 5ft above the ground and a creature 7ft tall 20ft away is providing the cover you should still be able to cast fireball 9ft above the ground, 40ft away.

Something to raise in you civil chat.

 I'm guessing you meant AOE but I love the idea of the 9th school of wizardry being APE magic

Quote from SwiftSign>>

Why are you intent on arguing with me?

You keep asking me questions...

It depends on the monsters involved. I’m not suggesting your slinky unarmored adventurers are in the way, but a well armoured Goliath and another bulky creature? Sure.

With two “normal” monsters in the way, they’re not making a solid wall, hence me saying I’d rather go three quarters cover, but I understand the argument that with them moving during combat might make it impossible to aim a spell or arrow through their spaces. Combatants appear static when using a grid, but remember they’re constantly moving within that 5ft space and clashing against each other. 

 The creatures in the OP blocking line of sight were his own party members. None of the PC races I know of are 3ft+ wide and barrel shaped. Closest is a dwarf maybe lol. But even they aren't that wide, plus, since they're less than 5ft tall, you could just shoot over them.

If a DM wanted an arbitrary but easy to administer rule for this, I could get behind:

• Draw a straight line of effect for a spell or ranged attack as normal. For every space this line passes through occupied by a medium or larger creature before reaching the target, apply one step of cover (e.g., half cover for one creature, three-quarters cover for two creatures, or full cover for three or more creatures). 

The effect of this would be that you can shoot over or through tiny and small creatures just fine, even if you yourself are small, just trying to make this as simple as possible... it would be probably more satisfying, but less succinct, to say "occupied by a creature that is the caster/attacker's size or larger" instead. Large and Huge creatures will apply more cover not due to their size alone, but rather due to having more spaces they occupy that your attack tries to pass through, but if you're just nicking their corner, than they wouldn't provide an excessive amount.

I think that this would be easy to anticipate and arbitrate for players and DMs alike, if using a battlemap. 

Quote from quindraco >>

Quote from Wtfdndad >>

We use cover modifiers pretty heavily. We also play on a VTT so, we can easily see how our movements affect line of sight. I would advise both strategic positioning and picking some spells that work off something other than aimed spell attacks or DEX saves.

Picking other offensive spells wouldn't help at all.  The targets aren't getting half or 3/4 cover (and aren't getting a bonus to AC or Dex saves), they're getting total cover, which blocks all spell targeting - forget about switching away from Dex saves to something like Int saves, the enemy backline is also immune to magic missile, which isn't an attack, literally can't miss, isn't saveable, and accordingly can't be shrugged off even with legendary resistance.  Strategic positioning might help, but assuming the math is 1 intervening creature = 1/2 cover and 2 intervening creatures = total cover, minionmancy would help even more - just bring your own total cover to the party that steps out of the way when you don't want it and steps back into the way when you do.

 Ah yes, my bad. I was forgetting that it would also assume that the DM had reread the rules on cover modifiers and understood them.