Finally, I got to 300 stars. This took quite some effort since the difficulty level has been considerably raised on some of the levels as compared to 2014 when the quests first came out.
If anyone is still trying for 300 stars from this game, this is what I can suggest from my experience:
Regarding upgrades - I would suggest first getting the dragooner (2 dark soul crystals), then the assassin tower + flycutter upgrade (7 crystals total), then the blizzard magic upgrade (2 crystals) then the witch tower + tornado upgrade (7 crystals total).
Dragooner is an awesome all-round tower with great DPS and AOE to boot.
Assassin + flycutter has good DPS and can deal with swarms of mobs. It is pretty much necessary to beat Stage 3-2 (Fauna Forge) Heroic, and comes in very handy against flying enemies later on in stages 2 and 3.
Blizzard is a great magic due to its AOE and good damage. The other 2 magic upgrades aren't all that useful if you're only interested in getting the quest, as the militia are basically just used to delay mobs for a few seconds, while the lightning spell only hits 1 target.
Witch + tornado comes in handy in some of the lava and desert stages to deal with armored enemies. It doesn't come into play nearly as much as dragooner or even assassin, but it's incredibly useful against Stage 3-6 (Vulcanland) Heroic.
Regarding fortifying - For the first 2 stages, you could really just upgrade all the towers equally, as the levels aren't very hard. Fauna Forge Heroic is probably going to be the first major obstacle you face. I beat it with arrows at level 29 and assassin + flycutter towers, though in retrospect I guess I could have done it with lower level towers.
Up to this point, I had barely used mage towers at all so I doubt the need would arise for you, until you hit the second major obstacle, Vulcanland Heroic. This level forces you to use only mages, and is just absurdly hard, so once you're done with Fauna Forge heroic I would say just focus on mages alone (maybe raise cannons a bit if you're stuck prior to Vulcanland). I needed mages at level 35 and witch + tornado towers to beat Vulcanland, and even then it was a struggle.
Once you cross that, just keep raising cannon level and the rest of the 300 stars should be a breeze.
For magics, I suppose hail/blizzard should be the primary focus, then lightning, and then militia.
Also at the start of this thread a lot of people are promoting the sniper tower + .50cal upgrade. I would strongly recommend that you avoid that altogether. I had it, and it helped me during the first 2 stages (which were easy anyway), and then I didn't use it at all for stage 3 and 4. Dragooner and assassin + flycutter are far superior to it for the first 4 stages.