Nop. Im talking about "im a lancer so i should be able to take one or two hits from bosses". I think thats pretty reasonable, right? when you play with a tank, you expect to last longer, right? you expect to be able to take a beat, right? whats the difference? or do you play with different classes for no purpose at all?
A oneshot mechanic is still a oneshot though and it should be no matter the class. If you are able to survive it it doesn't deserve to be called a oneshot mechanic.
About the second boss in RG: When it runs to a stone you are not supposed to stand in it's way and that attack ignores your block if I remember correctly. Any other attack shouldn't oneshot you. Most of them may kill after all their hits but normally you should be able to get your block up so that you don't die.
In TR it definitely matters which boss. On the second boss you need to stand in the right laser which is the color of your bubble. Also that mechanic does 100% HP dmg. So Kaias or Iron Will can save you at full HP. If the group failes the goblins at the last boss they will nearly oneshot you or maybe even oneshot you. If you aren't oneshot from them theres a good chance the boss will kill you.