In old version I had ALOT of trouble on the last boss, until I found two helpful tips.
1: Play 2 player, you can never die.
2: Quickly grab the electric vial, jump down back to ground level. Now face the wall (not to close) and shoot heaps of electric bubbles. When he moves in range pop 1 bubble, this will make a chain reaction popping them all. This should hit him like 7 times with one hit. You will know this is working because soon he will start flashing, after some more attacks he will turn into a bubble. Make sure you pop him otherwise hell come back stronger.
Remember though. Avoid is bottle and if it's 2 player make sure both people are alive before popping him. Otherwise it'll say "come back with your friend".
Hope this helped
Collect all letter without dying to spell "EXTEND". On old version it will give you a life, on new version it will skip you a few levels
At the Bubble Bobble New Title screen, press Right, R, Left, L, Select, R, Select, L
At the Title Screen, press L, R, L, R, L, R, Right, SELECT
At the Title Screen, press SELECT, R, L, Left, Right, R, SELECT, Right
At the Title Screen, pres Left, R, Left, SELECT, Left, L, Left, SELECT