Will it be easy to fix a dented back casing?

Dents can result in material stretching that you won't be able to hide completely without the right tools (even then sometimes it doesn't work). If you want to try fixing it yourself you can disassemble (assuming you have the skills or can follow the videos) and attempt to remove the dent using a few microfiber cloths, a flat sturdy surface, and a rubber tipped deadblow hammer.

If the dent damaged your speaker, it will need replacement. If it is just holding the speaker and preventing it from vibrating properly removing the dent should fix it but may degrade the performance.

Note: The deadblow hammer is very important. A normal hammer will not work and could cause further damage.