This video shows how to fix twrp boot loop problem as well as how to unbrick hard bricked mediatek devices. Although the tutorial is done in Moto E4 Plus, The same process is applicable to all mtk devices.
Tools required are SP FLASH TOOL: https://drive.google.com/open?id=1i2n...
FIRMWARE (Moto E4 Plus)(XT 1770) : https://drive.google.com/open?id=16Nv...