1) Silence it
2) Kill it with direct damage
3) Crazed Alchemist or Stampeding Kodo
The thing about Lightwell is that it's a very dangerous card if the Priest establishes board control, but by playing it, they lose a lot of tempo. It has no offensive stats, so if you can kill it, they lose board position. Silencing it is more effective than most units since, when silenced, it's worthless unless they buff it in some way. As long as you push the board early, Lightwell really shouldn't cause any issues since either the turn they play it or the turn after, you'll kill it.
Another minor trick for Lightwell. The Lightwell appears to heal a random damaged friendly character, so if you can't kill the Lightwell, the next best thing is to split your damage between the hero and Lightwell. If you entirely damage the Lightwell, then it's guaranteed it will heal itself. If you put 2 damage into the Lightwell and 1 into the hero, however, there's a chance it'll heal the hero instead, which is obviously preferable.
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