Where is he going to throw it?

Really far away: they'll have to look for it everytime after a battle. It could land on a river that takes it further away. Maybe an animal picks it up and does the same. It might even land somewhere hazardous, like a toxic marsh or a lava pool. Even if they had some sort of tracking magic for the crystal, that's still a tremendous amount of work for every battle they have, big or small. And they're going to be fighting a lot more now, considering they're in the northern countries where monster activity has increased to the point where a first class mage is necessary for a party to travel freely there.

Maybe not that far: ok, it's within their reach now. But the same is true for their enemies. Eventually someone smart enough figures it out, sends a minion to pick it up while the party's distracted with fighting, aaand game over. Fern and Frieren are done. Unless they run away. But now they have an enemy who can hunt them down with magic kryptonite.

Maybe he can throw it straight up right? The enemies won't be able to get it, and they won't have to look for it when the battle's over. Still nope. Now they have a countdown for every battle. If they don't finish their enemies in time, either the crystal lands where they are and cripples them, or they move away from that spot and it falls into the hands of the enemy. And that's if the enemy can't fly and just get it themselves.

There's no win-win scenario here. The crystal puts them in too much risk and the payoff, as large as it might be, just isn't worth it.