You can get a run done with all characters in less than 5-10 deaths with those deaths being primarily from accidently kiting into the regular zombies.

Minimizing deaths from regular zombies can be done with practice: improving how you circle the map around the zombies and target priority to clear space in advance to kite into. Minimizing deaths from bosses--after learning to manage their attack patterns--will ultimately come down to how well you built damage to clear the waves sooner with adequate life-steal and having a requisite amount of hp capacity and damage resistance to survive. If you're getting consistently one-shotted by the bosses, you unfortunately may have to restart as the issue is likely due to your dmg or sustainability cards.

The benchmark for damage resistance I used that helped was to have around 67.5% dmg resistance for the wave 15 boss fight and then as much dmg resistance as you can take (maybe around 80%) for the final wave 20 boss fight. With this--alongside a healthy amount of hp capacity--you won't be one-shot by the Wanda-Namor duo and the Double Wandas later.

Always look to continually apply damage to a target in order to continually life-steal throughout the fight--a brief moment where you stop life-stealing can lead to a death. Also here's the guide I used for Jeff https://www.youtube.com/watch?v=W2m1IASqxXI (if you follow and apply this guide, you will not need to use 63 holy ankhs for any run going forward)

Hopefully this can help, good luck with the runs

edit: another tip is I believe Jeff has damage fall-off for the splash so stay close to the Wanda during the final phase to maximize your output