This is the exact same strategy I used to get
Personally I did this on the suburbs using the Auroch from the apocalypse dlc pack. It has the same stats as the bus but it has a 2 in speed plus it handles loads better. I liked the suburbs because they didn't have a lot of stuff to get hung up on. What I did was drive down the center stage back & forth in sort of a pattern while picking up whatever upgrades came my way & constantly checking for health to show up. At first I'd shoot my way through everything then later on when a ton of those green exploding guys showed up I would start saving my ammo & use it to get rid of them.
If you give this solution a thumbs down please inform me why in the comments section.