Here is the second video of compromised audio, so you get to see the set I used on the elder trio in action here.
For Pukei-Pukei, he still doesn't really have that much going for him. If you want a poison weapon for Kushala, you will need either him or Rathian (worst case scenario, both). Otherwise, you could look into poison knives or other poison bombs to inflict the status. I stand by my comments about poison being the weakest status effect in the game, and even Kushala may not be enough to justify seriously using one of those weapons.
As for Rathian, I don't really notice anything different between her low and high rank. I've fought her so many times that there may be something that I just tune out by now. She will not stay in the air as long as Los will, so Flash Bombs are not required, but could give you some additional windows to attack from.
If you don't need poison gear, move onto the next part and see if something there happens to be what you need.
...
https://www.youtube.com/watch?v=v_Q71IjdBLo