I had a lot of problems while installing Kali Linux. The biggest one was: I couldn't boot into the GRUB (had to use rEFInd). I have, however, solved this problem. I searched EVERYWHERE for finding a way to get GRUB to show up in the boot order, but to no avail. I started fiddling around in the BIOS and found out that you can tell the computer which UEFI it can trust. Here is how you do it:
1. Power-off computer completely and reboot into the BIOS (for me, I hit F2).
2. Switch over to the TAB titled "Security". Scroll down to "Select an UEFI file as trusted for executing" and click on it.
3. Go into the BOOT directory and find the Kali folder. Enter into the folder and select the grub efi file.
4. Finally, go to the BOOT tab and turn off secure boot and push the Kali Grub up ahead the Windows Boot manager in the boot order. Viola! Du bist fertig! Exit and save, and you are finished!
Disclaimer: Not all devices have this option! I found that the Acer laptops do.