Blood Ritual is a great way to Enrage units. This way, you can plan the perfect moment to Enrage a large group and sick ‘em on the enemy, while also boosting their defense, attack speed and movement speed. If you’re not using it on a unit with Enrage, then you should instead choose to use it on a unit with high HP, so they get the buffs without taking too serious of an HP cut.
As for the three buffs themselves, the defense buff can work together with Turtalon’s aura, while the attack speed buff combo’s nicely with Call of the Wild. Finally, you might want to try using Blood Ritual on Trample, who has both an Enrage bonus and an active heal.
No surprise here, the best way to deal with units that have undergone a Blood Ritual is by defeating them as soon as possible! Definitely try not to give your enemy the chance to heal! Consider neutralizing their defense buff by lowering their defense with Venomite or AP Strafing, while utilizing burst damage with Landmines or Painlings to take ‘em out!
Otherwise, you can also use a tank, such as Pulse Tank or Imperial Guards, to buy you time while you damage them with ranged units like Marines or Mad Axes.