I have not played Raid Mode in Revelations 2, but I have in the first game. It's basically like a dungeon crawler, you do defeat enemies and that's the main focus, but there's a bit more to it. Some levels in Raid Mode were linear, others had multiple paths and endings... The basic idea is that you start and have to make your way to the goal post, in-between you and your goal are rooms with enemies, traps, and the like. The enemies can have weird and crazy buffs, like some enemies might be 4x taller, or some might be tiny, or have super speed, or they might be on fire, or they might suck you in, or have shields... Clear all the enemies in a room, and you get a key. The key can be used to open a locked door. Besides enemies, there's also loot chests to find and custom parts to collect.
And some stages changed the pacing, like one is just an enemy survival mode, another had hardly any enemies and mostly was maze-like underwater tunnels. As you do all of this, you gain experience, level-up your character, and get loot for you to use and parts to customize your weapons and such.
I hear in Raid Mode 2 they have new additions, which include character-specific skills, and challenges to complete stages under certain requirements to earn medals for more unlockables. These challenges differ from stage to stage, but include things like don't heal yourself, use a certain number of bullets and no more, complete within a limited time, and so forth.
It can be pretty addicting. Each stage also has three difficulties, and each difficulty, if like the first Revelations, doesn't just make the same stage harder, but completely remixes the level with different enemies, positions, traps, and more.
