Thanks @HenryThePear .
Unlike the normal Arena matches where there is not enough Zombies to kill so with high level plants we could have killed them too fast and causing ZDL drops, this Zomboss match other the other hand, requires high attacking power to keep Zomboss to be stunned thus preventing him from doing his 'dance' annimations. With level 8 plants (or lower plant levels) we have to use the mint to boost the attacking power to achieve this and with level 10 plants, it is a little easier.
IF your focus is on the high score, then getting zomboss to the 33rd health bar by killing all the other zombies Appearing on the Arena ASAP should allow you to break 31M.
IF your focus is on the 34th health bar, you will need to focus on Zomboss, try to attack him and stop him from jumping around. I suggest try dropping Pea Vine onto the power tile and where he lands. I have quite a few 34th health bar games with only 31M by doing this.
Now the challenge is, try focus on for both the high score and he 34th health bar, this requires some RNG luck.