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Later on, you can get cancerite booze, which can heal 900, but the user gets knockdown. Cancerite boozes are actually a great healing item for a long time. They are also good for trading. Try to farm for them in the Diadem area with treasure lowdown of course.
Then, you may have access to elixirs, which heal full life at the cost of 1 MP point. When you get to a certain point in the game, you will have the Book of Mana, which is OP. Only Guillo can use it and it heals full life of all party members, but it doesn't revive. The Book of Mana costs 1 MP as well. Finally, Fate's Kiss, which you get near the end of the game. The thing isn't as good as the Book of Mana in my opinion. It revives everyone and heals everyone to full hp, but it costs 3 MP so it is very costly. Again, only Guillo can use it. As you can see, protecting Guillo is very important.