a few questions:
- you mentioned you are running an ego twist. what voltage was it running at when you first had burnt tastes?
- have you tried to lower the voltage to see if the problem persists?
- metallic taste can sometimes be related to poor wicking (distinct taste of coils), what type of juice are you using? what's the pg/vg ratio?
in all honesty it can be frustrating but it sounds simple enough to fix, and I personally don't think you need a new tank just yet
ps - I can't think of any reasons or ways for your battery to ruin a tank. however, once you have a couple burnt hits, it is possible for the rest of the juice in your tank to get 'contaminated' with the nasty taste. even after you switch to a new battery, it can still taste nasty. it depends if juice is getting burned (voltage too high) or if it's just poor wicking (juice too thick / very dark). poor wicking normally doesn't affect the taste of the juice that's left on the tank. once it wicks properly it goes back to normal.
one thing that you might be able to test, clean everything up, replace the head/atty, test it on the lowest voltage setting or better yet, on your friends battery. so: new juice, cleaned tank, new head, lower voltage. if that doesn't fix it, I am out of ideas.