Simply put, this game, mechanically, plays almost identically to the mercenaries mini game from Resident Evil 4 and Resident Evil 5. It includes 8 characters, and 1 alternative costume for each character. Each character comes with their own loadout of weapons, and there is also a bonus set you can use play coins to purchase (Around 20 Play Coins if I recall correctly) bringing you to a total of 9 weapon sets to choose from. You can buy each set with Play Coins to make them available for all characters, or you can just use the character that owns that weapon set. Each character also has their own set of Melee moves, taunts and animations creating a greater sense of uniqueness to every character.
You unlock more characters, and eventually their alternate outfits, as you play and successfully complete levels. This can all be done in single player, and most of the later levels (Those that are not tutorial levels) can be played in multiplayer either through local play or over the internet, with the choice to specifically join a friends game rather than a random game. In my experiences I have had no issues with the netcode playing through the whole game with a friend whom I bought a copy of the game with the sole purpose of playing through the entirety of the game with.
The game adds a 'skill' system, which introduces the ability to equip three unlocked skills on your characters. Your skills carry across to all characters and will level up through play as you use them, generally gaining a special bonus passive for having them equipped when at maximum level. An example of this is the medic skill (Which boosts how effective you are at healing yourself and your partner with herbs) granting a small amount of lifesteal from a critical hit when at Level 3 (The maximum level for all skills). There are a wide variety of skills ranging from boosting attack power of certain weapons to adding lightning damage to melee attacks or granting extra damage for jumping down onto an enemy, opening the game up to more customisation and variety of playstyles than any previous rendition of the mercenaries has.
The difficulty of the game as you progress is perfect for any veterans of Resident evil 5's mercenaries game, with the EX-Levels providing a significant challenge even to experienced players. The stages in the game are recycled locations from Resident evil 4 and Resident evil 5, so don't expect new locations or settings. That being said, the stages are varied in their layout, size and aesthetic appearance. There are 8 stages in total, 5 of which come from Resident evil 5, with the remaining 3 coming from Resident evil 4. The enemy variety in the game is sufficient enough, but there is certainly a missed opportunity in the exclusion of some enemy types, such as the plaga-infected villagers (ganados) from Resident evil 4, which is made even stranger when considering that one of the stages is the village that those ganado enemies occupied in the game.
Overall I found this game highly enjoyable and have garnered well over 35 hours of play from this game so far, and I do see myself continuing to play a few rounds of this with my close friends in the future. If you were a fan of the Mercenaries mini games than this game is for you, and I hope you get the same enjoyment from it that I have.