The third guardian is the Legendary Encounter in the 3rd area, which will be moderate to diffcult depending on your level and experience gained in getting here. The fight will be against the Mistbearer or the Horned Warden, each of whom will have 2000HP. In my opinion, the Mistbearer is the easier version of this encounter. The Warden will summon 4 totems that each steal a card from your deck, locking it from use for a while. This is a great problem in smaller decks, though you probably won't notice in a large one. The Warden himself has a high level of Armour and his totems like to buff his Armour and damage when they aren't stealing your cards. Unless the cards stolen are truly essential, focus on the Warden and just relentlessly attack him, playing enough defensive cards to avoid damage when he is attacking. Lowering his Armour can be a great help, so cards that have this effect are invaluable, as well as Acid Flasks and Bombs.
By comparison, the Mistbearer is basically a single big sack of HP that deals moderately heavy damage. It will attack multiple times per turn, though normally at the same target. Again, lowering its Armour is invaluable, so bring your cards and Acid Flasks and Bombs in order to make it go down fast. The faster you can kill it, the easier it will be.
In my opinion, assuming that you have been fighting every encounter on your way here and are thus Level 20, the easiest class to complete this with is the Summoner. Summon a Golem to absorb the damage and a Wyrm to deal damage to the enemy. If you have the Warden variant, an Abomination may also be necessary to deal with the Totems. Buff the Wyrm's damage Armour reduction and Promotions as much as you can, though make sure to also keep a Barrier around the Summoner and the Golem in good shape/summon a new one, otherwise it can go very wrong, very fast. Once you have a Wyrm at around Level 10, it will be dealing 100's of damage with each hit and the HP of the guardian will just evaporate.