Extra Hard Wild West Level 25

I have not played these quests since the reward was further nerfed from 80 gems to 25 gems (originally the rewards was 100 gems with 10 levels).

For the fast moving chicken, I suggest you either use Wall-Nut or Spikeweed or both with the electric fence to stop th chicken.  Alternatively, if you have Hypno-shroom spare (I guess this should be a big and bulky IF) you could also use it to block the chickens.

Unfortunately, it’s a conveyor level so, can’t choose own plants

All the plants I suggested are available on the conveyer on this boss level. 

There is a known strategy to beat this level:

https://answers.ea.com/t5/Plants-vs-Zombies-2/Extra-hard-Wild-West-day-8/m-p/11422984/highlight/true#M43573

Levels of the given plants don't really matter, actually mastery level (or boosted) Hypno-shroom can even be a bad thing - you can't give PF to a boosted Hypno-shroom to block Zomboss' charge, and it can be hard to tell whether Hypno-shroom is boosted or not.

The strategy above doesn't always work, for me the failure usually happens in phase 3 when I got overwhelmed by too many spawned Gargantuars. However, with some practices, I can beat this level most of the time now. For spawned Gargantuars in phase 3, my 2 tips are:

1. Use PF on Iceberg Lettuce to slow down Zomboss and spawned Gargantuars, hopefully newly spawned chicken wranglers can be hypnotized and release hypnotized chickens to defeat these Gargantuars.
2. When Zomboss is about to charge, give PF to an appropriate plant at the correct time to let Zomboss' charge kill these Gargantuars but block it from destroying your plants further down the lane, especially the Spikeweeds in the first column, which are critical to defend zombie chickens not killed or released by Zomboss' charge.