The first boss you will encounter in Loop Hero is the Lich and in order to engage in combat with this boss you must play until The Boss Meter (underneath the Day Meter) progress bar in the top left corner of the screen is completely full. Once you have met this requirement the game will spawn The Lich's Palace.
Once this has happened the Lich which is in the form of a skeletal specter will drop Lich Palace structures on any empty tiles around your camp which will give the boss an additonal 5% health and damage boost. Loop Hero will give you an Oblivion card when you face the first boss, this helpful single-use card can be used to purge any tile and return it to a Wasteland square. The Oblivion card is unlocked early in the game, and although they are useful for destroying Goblin or Bandit Camps it is far more beneficial to keep these rare cards in your deck and stockpile them for the Lich's Palace tiles to make defeating this boss easier.
If you have not managed to collect enough Oblivion cards to clear out all the Lich's Palace tiles you should surround your camp with other tiles in order to prevent the Lich from building there. Ideally you should surround your camp with Hungry Groves, which will instantly kill the Lich when it reaches below 20% health. You will be able to create these tiles by placing a Blood Grove next to Grove and using the Oblivion card to destroy the Grove. You can tackle the actual fight with the Lich boss however you want with the vast number of card combinations that are available but the key to defeating him easily is to abuse the power of the Oblivion card.
Take a look at our Best Card Combos guide to find out how you can use the cards in your deck to farm more resources and acquire better gear to help you defeat the Lich.
We have questions and answers related to this topic which may also help you: