“Oh, so it’s just like Halo’s Firefight or Gears of War’s Horde mode,” I thought to myself somewhat apathetically as Sledgehammer explained their new co-op mode to resident IGN first-person shooter pro Alfredo Diaz and I during a recent visit to the Call of Duty: Advanced War development studio.

Every Call of Duty these days has its “curveball” multiplayer mode, of course. Black Ops boasts the undead-infested Zombies, Ghosts trots out its extraterrestrial Extinction, and Modern Warfare packs Spec Ops. On paper, Sledgehammer’s multiplayer alternative to traditional deathmatch looks a hell of a lot like the humans-vs.-AI mode that Halo 3: ODST and Gears of War 2 pioneered five years ago.

Here’s the thing, though: Exo Survival, as it’s called, is incredibly fun in a uniquely Call of Duty way.

With support for 1-4 players (including two-player split-screen), Exo Survival pits your crew up against infinite waves of PMC pains-in-the-ass, from cannon-fodder grunts to pesky drones to ravenous attack dogs to dash-happy Exo-wearing equals to mech-suited Goliaths. The action takes place on the game’s large complement of multiplayer maps, rotating through each in a series of four tiers. New tiers (and thus, new maps) are unlocked with playtime. Completing 35 rounds (not necessarily all in the same session, by the way) grants Tier 2, 75 rounds nets access to Tier 3’s maps, and finishing 100 rounds will unlock Tier 4.

Unlike Gears’ Horde mode, Exo Survival doesn’t feature a “boss wave” every tenth round. Instead, random objective-based rounds will spring up, such as collecting “intel” off of a certain number of corpses in a certain amount of time. Succeed and you’ll get extra upgrade points (more on this in a moment). But -- and here’s where one of Exo Survival’s clever design decisions kicks in -- fail and you’ll suffer a penalty. For instance, you might suffer a “system hack,” in which your screen goes blurry for a brief period of time during an onslaught. Or you might be temporarily stuck with a limited number of weapons, or have sentry guns dropped into the map and trained against you for a round. In other words, you’re genuinely motivated to not fail. It adds an extra layer of tension, risk, and reward/relief.

Alfredo and I played on the “Biolab” map with no restrictions. “Let’s see how far you can get,” the Sledgehammer team said sadistically. And so, sitting side-by-side, we made a pact to always stick together. We each chose one of three classes: Light, a faster loadout with less firepower, a Heavy, who’s slower, or Specialist, who’s in the middle (for what it’s worth, I initially tried the Heavy but couldn’t stand feeling so encumbered, so I switched to the Specialist). As the initial wave approached, we holed up inside a tube-shaped room-connecting tunnel and stood back-to-back, not unlike a pair of muscle-bound bros-to-the-close in an ‘80s action film.

It certainly helps if you each pick different classes so that you can complement each other’s firepower. For instance, in the beginning I could protect us up close with a beefy shotgun, while Alfredo could thin the enemy’s numbers from afar with this longer-ranged weapon as a Specialist. And yet, we still did great when we ended up both playing as the jack-of-all-trades. We wiped out everyone in fairly short order as they swarmed toward our bubble-tunnel death trap, and the early rounds ended quickly.

Between rounds is where Exo Survival’s other big piece of intelligent game design shines through: post-round upgrades. You’ll earn upgrade points for kicking ass and accomplishing your dynamic objectives. To use them, you’ll have to quickly run to the upgrade station on each map (there’s also an armory locker where you can swap out or upgrade your guns) and spend those points improving your Exo suit. I pumped a lot of points into passively beefing up my armor and machine-gun damage in order to make me more generally effective.

You’ll also fill a support bar on your HUD as the matches roll on, earning you bonus support drops when it fills.

As we crept into double-digit waves, the intensity and difficulty quickly began ratcheting up. Nimble drones proved to be a particular annoyance, buzzing around while being very small targets. Worse, enemy AI PMCs started dashing around like superheroes with their own Exo suits, using verticality and smarts to make our lives extra hellish. In fact, they were so welcomingly challenging that I hope this supreme intellect finds its way into the AI at some point during the single-player campaign. We managed to fend off a wave of Goliaths and a few more, reaching Wave 16 when we both finally died. For what it’s worth, Sledgehammer told me that the highest wave any of their tester groups has been able to reach is 65 -- and no, you can’t pick up where you left off once you quit.

A few other fun facts about Exo Survival: it does, in fact, feature a down-but-not-out system where you can revive your fallen comrades before they actually die, which Alfredo and I each put to harrowing use a couple of times during our hands-on time. Meanwhile, your character progression is persistent, making the mode more appealing to keep coming back to. And thankfully, matchmaking is supported in addition to private games, so you’ll always have potential pals to partner with in case no one on your Friends list is online.

Speaking as someone who’s a bit intimidated by Call of Duty’s traditional adversarial multiplayer modes, Exo Survival may be just the thing I’ve been looking for in order to get my Call of Duty multiplayer fix without the confidence-crushing humiliation that comes from being stomped by human rivals. But more than anything, Exo Survival is a pleasant, well-executed surprise that may prove to be my favorite of all the Call of Duty franchise’s “curveball” modes.

Ryan McCaffrey is IGN’s Executive Editor of Previews and Xbox Guru-in-Chief. Follow him on Twitter at @DMC_Ryan, on IGN, catch him on Podcast Unlocked, and drop-ship him Taylor Ham sandwiches from New Jersey whenever possible.