"Monster allies" refers to creatures that are typically seen as scary, powerful, or intimidating, but are instead working together with someone to achieve a common goal or objective. Full definition
Normally I do not mind grinding for experience, but when you are trying to create better weapons, armor, special moves, or creating a party of monster allies it becomes tedious. (nintendofuse.com)
Upon entering each arena in a level you'll activate a Test of Faith, which act as modifiers, mixing up the gameplay by doing things like making it rain bombs, spawning monster allies near the Altar, giving you Infinite ammo, making the enemy wave tougher, turning on friendly fire and even changing player's weapons ever few seconds. (wolfsgamingblog.com)
This comes in the form of summonable monster allies, spells that can be cast by pressing the right shoulder button, and the ability to jump around to any character within the party you're in by tapping the left trigger. (playstationlifestyle.net)