I could see a use for this mob attacks do 75% less but everything else is the same there HP and timing.
This would very much let people that can get timing down to get to phase 4 of a battle easily to get to that to get it down vs fighting with it.
Think back to t9 when it was current content.
Great our groups can make it to final phase. but we wipe. But we only make it to the final phase 30% of the time. Err RAGE QUIT. Replace new person teach them the fight. make it to final phase 25% of the time for the 1st week.

Something like this. See fight get good at fight without having to freak out about wiping, and understand what is happening.
Finally reward weak like 1 current tomb 2 older tomb