Important Background info on Masochist mode: Masochist mode exponentially inflates the difficulty of play in that previously normal hits from mobs now have the capability to one-shot you… And normal hits are not telegraphed… so for any previously hard hitting attacks, that are now one shots - you need to be aware that they were not designed as one-shots or intended to be one-shots. Because of this and a lot of other things, the devs have expressly stated that they would not take any “balancing” related feedback into consideration for masochist mode play… They have no intention of changing anything based on masochist mode feedback.
Now, onto your two boss issues…
The Light Arbour boss (fight guide: Lightless Arbor Dungeon Boss Guide (In-Depth Mechanic Explanation))) can be dealt with if you understand the mechanics… However, you are playing under masochist mode, so depending on the dungeon tier, things that are intended to be optionally tanked by some builds are unlikely to be tankable for you…
Lagon fight… There are two issues here… The smashing wave is likely to have killed you because of its increase damage in the mode you play… The issue about having ward but zero health has come up a few times and is usually attributed to the UI updating incorrectly - i.e. you died with both zero health/ward but the UI updated after the fact with something like ward or health regen/on hit. This has happened in a fair number of cases - usually with hard hitting boss deaths (not just Lagon). The devs are aware of this issue but it doesnt indicate anything wrong with the fights specifically.
It is very hard to take your comments about one-shots, difficulty curves or anything really into consideration if you play in masochist mode - again, the devs have said that they will not entertain balance issues / feedback from this mode.