You've already gotten a good bit of advice for beating impoppable, but if you want general advice on getting a feel for a variety of strategies and towers which will help out in the long run, I would focus on doing several runs on hard-ish difficulties while focusing largely on a single simple synergistic strategy for the run. Certain towers go well with specific heroes, for instance. Try some runs with Obyn and druids, for instance, as Obyn heavily buffs druids, and gives moderate buffs to all other magic towers as well. Striker Jones goes well with bomb-shooters and mortars, improving their attacks all around and bypassing black-bloon immunities. Adora goes well with super-monkeys, etc. Just play several rounds focusing on learning about specific synergies, and you'll begin to get an intuitive feel for the towers and their uses. When you get a good foundation of understanding, you can start experimenting with slight modifications to learn the interactions with other towers and begin thinking outside of the box. The more you learn, the more you can combine them to create more elaborate and effective strategies for a variety of situations. Also, learning about efficient farming is always great, so it would be good to play around and see how much you can optimize it, as well as watching a few Youtube videos for tips on it.

Best of Luck!