The Lava Guardians are a melee Orc enemy that is extremely tough and has massive health - about as much as a Boss. This enemy can only appear in Onslaught floor 28 or higher, or the Demon Lord Incursion.
Lava Guardians deal high melee damage and can spawn a fissure that erupts in a straight line directly in front of the guardian. If the lava guardian is attacking a blockade, this fissure is capable of damaging towers directly behind the blockade, instantly destroying them in many cases. If your hero is also hit by this fissure, you will be Knocked Up into the air and stunned until you land on the ground.
Whenever a lava guardian dies, a lava orb will be left in their place. Running over this orb with your hero will cause you to collect it and give you a status effect that boosts your hero's damage and movement speed. This buff will last until your hero dies or you swap to another hero (your original hero will not keep the orb in either case). On the Demon Lord Incursion, placing the lava orb onto the catapult also removes the buff.
Lava Guardians are another variant of Orcs and take on a lot of their traits - including a lack of movement speed. This makes crowd-control builds very effective, as the lava guardian's slow movement speed can make it difficult to escape. A Poison Dart Tower paired with an Earthshatter Tower is a great way to Petrify lava guardians and provide moderate damage as well.
Cannonball Towers or other single-targeting defenses are great, since lava guardians usually spawn alone or in pairs. Most single-target defenses deal more DPS than multi-target or AOE defenses do, so it's a great way to counter their high health. However, other enemies usually spawn along with lava guardians, so you will most likely want some form of AOE damage as well.
In Onslaught, only certain lanes will spawn lava guardians, and many of these lanes don't spawn any until later waves, so you might not have to prepare for them if you don't have enough defense mana. Hold your shift key to see if one will spawn or not.