Rather than having a standard attack, Beast Riders have a long range "spit" attack that, notably, can reach from the enemy hero to the right hand side of the screen, potentially avoiding the usual build up of enemies that happens when you have the hero cornered for awhile. On the downside, this attack is broken into two 0.5 power attacks, meaning they only deal their full damage but apply defence to it twice, thus their otherwise impressive attack aptitude is effectively 19.5% as far as determining what units they can deal with goes. While this may improve if a tanky thunder unit with a poison weakness comes out in future, the Beast Rider's best feature currently is as a surprisingly effective tank, with many hard hitting units having poison status that renders them almost harmless to the Beast Rider.
Just summoned enemy spawns. Thunder aligned ranged units. Poison users.
High defence units and tanks without a double Weakness/Element to exploit.
Wind units that don't use poison.