Post-Apocalyptic Granny x Mecha Action "The Doomsday Witch and the Doll" vol 1 by Shinichirou Takada

Sci-Fi Action set in a post-apocalyptic world after large-scale disasters and wars. A female police detective is trying to uphold order in society when one day she's saving an old lady who is having some troubles with gang members. But soon after this seemingly helpless old lady turns out to be a legendary marksman and on top is using a powerful mecha assistant to get rid of her foes.

The manga is recommended by "Mobile Suit Gundam Thunderbolt" creator Yasuo Ohtagaki!

(Shuumatsu no Majo to Ningyou)