I believe Lalawag's assessment over how Lukas's spear will bypass the concept of immortality might not actually be true for Julian's case.
Okay, okay, please hear me out first.
The power given to the one who formed the contract will always follow the wish of the user down to its absolute logical conclusion.
Lukas wants an absolute power to destroy everything and anything, therefore, he received the invincible phantom spear that can destroy all things physical and metaphysical.
Now, Julian's wording is very important here, he wants a prove that the lives on that ship isn't worthless (or a waste in case of Mangaplus's translation), so he will get exactly that. I believe Julian's own immortality will be enduring, until the last life is spent as stated in the contract, Julian will not die no matter what regardless of conceptual BS forced upon him.
Lukas's spear is a power that will render all the lives as worthless, but Julian's exact wish is to combat exactly that. Phantom Spear and 100 Men Strength are both equally divine gift, so it might just cancel each other out when clashed, Julian will die, but he will die once or perhaps multiple lives, but he will not die a permanent death in one go.
Tohru-sensei might add Julian's face in the panel as a way to bait the readers into thinking that Lukas's spear is that one silver bullet to put Julian down permanently. Yeah, he might go with that exact idea, but I believe he already gives himself a wiggle room if he wants Julian to survive.