Simple modification that skips all rush bosses except the first one. Defeating Shield Plant will complete the challenge and trigger the achievement to pop out.
Your Best Time will be affected but this too can be edited or deleted from save file if you wish to tempt your luck RNG at a later point.

Either one works, second is just the first but performed locally.

Download edited 08boss1.pxeve[1drv.ms]and place it in
STEAM\SteamApps\common\Kero Blaster\rsc_k\text.

1. Navigate to above folder and open said file in any text editor
   
2. Replace last line:
   
   <dl60<fadO<plAc[warpOut]<trns[08boss2;pos1;IN]
   with:
   
   <dl60<fadO<plAc[warpOut]<taFi<trns[08result;pos1;Scene08]<exit

When done simply run game and beat truncated boss rush from end of Omake Mode.

This has no purpose but to remove impossible time and allow you to improve it legit. Or simply restore old time before before you ruined it.

1. Open profileX (X=0 or 1 or 2, corresponding to in-game profile list) found in STEAM\SteamApps\common\Kero Blaster\rsc_k\profile, in any text editor.
   
2. Unlike before this file has lots of scrambled data but our Timer is not among them.
   Find obvious looking clock string near the top and change it to your liking.

Format is XX'YY"ZZ (minutes'seconds"hundredths). For example 00'09"45.