Coda is ridiculously hard, the main way to actually improve with her, is just practice. Get to know all quick kills for bosses, ways to handle groups of enemies, and since you complete All Chars, you should already know how to kill all enemies without being damaged. I have only beaten Zones 1 and 2 with her, but the main thing is just practice. You will die A LOT in each individual zone, so much that it seems impossible. Then you get that one perfect run of the zone, that helps you beat the Zone.
You will eventually be able to beat every Zone, and possibly All Zones with Coda, it just will take lots, lots, and lots of time and practice. There is litteraly not much else to say. Also for vow of poverty, try killing enemies in the same square or area.
I just unlocked Coda, and I can't even finish 1-1 with her. About 90% of my deaths come from thinking I have two squares of range and walking. NOTE: This run is REALLY OLD! Go watch these instead: Coda Speedrun World Record (3:31.56):.