Onslaught mode is a welcome alternative to the standard raid mode; however, it could be improved.
The first is a mere tweak: in the trash mob phases, when time runs out, don't simply end fights in mid-blow; allow the player to complete the current wave, then end the wave. It's frustrating to have that one last wolf escape with a sliver of health 0.05 seconds before the final blow lands.
Second, onslaught mode simply doesn't work, conceptually, for boss fights. Of course, the standard raid mode never worked for boss fights either, for the same reason; it's difficult to keep killing the same boss over and over and take it seriously.
Therefore, instead of giving bosses a regular old health bar and a number of "lives," use the timer mechanic you've added with onslaught mode, and revert to the long yellow super health bar/percentage indicator from the standard mode, but make bosses invincible in a single fight. When the timer runs out, something like this would happen -
"Helion gathers his powers, and an explosion of dark magic hurls you back!"
"The guardian dragon beats its wings furiously, driving you back!"
- and the player would have to pay an energy cost to return to the fight. RPP would be gained based on the damage done: if, under the current model, a boss has 10k health per "life," every 10k of damage would earn 100-150 RPP (depending on the player's bonus), with a fraction of 10k being prorated (thus 5k damage would earn 50-75 RPP).
This would eliminate both the cognitive dissonance involved in repeatedly killing a boss, and also remove the niggling annoyances that onslaught mode has added while solving the old model's problems: in standard mode, past a certain point improving one's character has no effect; in onslaught mode, some debuffs are maintained while others are not, but it's not always clear why (poison from Toxic Strike sometimes vanishes after a "life" is eliminated, but sometimes it doesn't, especially if Toxic Strike is the "final" blow, but this is not guaranteed, while poison from Poison Cloud always continues, as does the Acid Spray debuff).
Players with stronger characters and more skill would keep the higher rewards they gain in onslaught mode, but game mechanics would be more clear. Conceptually, it would be easier to accept. And finally, boss fights would become markedly different from trash mob fights: the objective would be to pour as much damage onto a single target as possible while avoiding damage (or stuns and slows that would decrease the timer to no effect, thus costing more energy per RPP) oneself, but considerations of "overkill" wouldn't be relevant.
Of course, this system could be used in the standard raids as well as in onslaught.