Goblin Siege Roller
The Goblin Siege Roller is a tanky mini-boss first introduced in The Dead Road campaign mission which fires rockets at players and grinds down barricades. The Siege Roller moves very slowly, but does huge amounts of damage and mows down everything in it's way.
Strategy
The Siege Roller has a large amount of health and ~90% resistance from all sources from the front. It also does massive damage to all barricades, most solid towers, and Heroes from in front. The weak spot is attacking the engine behind, which does not resist damage. Use DPS heroes from behind to bring it down quickly.
It is not possible to distract the Roller from it's set path towards the crystal. While it is possible to slow and stun the Roller to some extent, crowd control is usually less effective than simply using pure DPS and strong walls. Keep repairing your barricades often when the Roller attempts to grind it down, and you can stall the Roller for as long as you have Defence Mana.
Like assassins, it is generally a good idea to stay away from objectives, as the Roller may damage the core from afar while trying to shoot down players.
There is an extensive documentation on Steam here.