When the wither's foot is inside of any block that has a hitbox or a boat, the wither's skull would explode immediately. If there's piston pushing blocks at its foot, the skull will explode at its foot position.
- build a contraption like this.
- summon in the wither and remove the redstone block before the wither finishes summoning.
- the wither's blue skull explodes at its foot and destroys the obsidian