How to win

[Orange Guise]

Set up a Stone tile.
Surround it with cheese.
Wait for lava, then dig out water to get obsidian and flood your dungeon.

Build a lightningrod on the stone tile.

Your dungeon is now a Giant Bug Zapper, and you win.

[ovni.demon]

If you want to get the "Free real estate" achievement and the 100 heroes killed: use stone ground on the access to the crystal, it stops the water. Then just summon goblins to dig all around (make sure that there is only one path to the crystal. I recommend to change the access to the crystal into stone to avoid getting lava there.

For the 100 heroes: at later level, make a corridor to your crystal in obsidian, one obsidian chimney and summon fire snake. Later heroes are immune to the zapping (except the knight and thief). You can also make a corridor with a chest at the end with spike and poison plants all around (dragon knight are immune to spikes but not the poison)

One detail that may make you lose: the wizards can attack you crystal at distance through the walls. So don't give an access to the heroes that goes near the crystal if you dig out a lot.

A last point: you may need to reset a few time if there aren't any easily reachable water; or if the lava spawn at bad place. At the start, I recommend to nearly destroy you crystal, upgrade, summon a goblin (fastest to mine/dig/build) and after that only skeletons. Don't summon skeleton too fast since the priority for work is bad. Dig until one case is left to reach water in the main room. If you can, do the same in the entries corridors. When lava arrive, spawn a skeleton to the nearest water and dig. If the lava block access to the water, use a imp but he is really slow.

[Orange Guise]

Oh, I already beat this game once.
Look at that. Thanks for the tutorial, me