All of them might be tired but that does not mean that the Physalia is not getting tired as well. The Power of Kindness is no something that can continuously be used without respite. The Physalia did not stop in regenerating its limbs which meant that it expended lots of its mana in the process.
The baby angel Raphael already told them that the weakness of its powers and that is the burden or cost to use it. The power to heal is not something that could be done easily without a lot of mana. Angels were not a race that could use healing magic naturally, but the Angel Raphael was created specifically for that. He was the only angel that is said to have the power to heal.
