The Lich is a vehicle that only appears once in the entire game and only ever in campaign. It is highly unlikely that it'll appear in Multiplayer since the player never gets a chance to control one even in the campaign.
For comparison, the Lich is very similar to the Scarab from Halo 3 or Reach if you replace the legs with a hover system. The outside of the vehicles is nearly impregnable but there are several ways into the ship. There are two slots on either side of the ship through which turrets can shoot. It is theoretically possible to get in or out through these slots. It is also assumed that if you could land on top of the Lich, you could fall down and gain access in this way.
The method used in the campaign is by using a gravity lift under the ship. This lift is usually used for troop and supply movement. If you take the lift up, it will deposit you in a ship with two layers. If you get to the upper level and head to the back, you will find a power core similar to that of the Scarab. It will appear similar to the Lekgolo (the worms that make up Hunters). weaved towards the center with a light. A plasma grenade or sustained fire should destroy this quickly. After that, the ship will begin to detonate. The best method of escape is by jumping out of the side of the ship with a Jet Pack to slow your decent.
