At a rooftop party a couple of blocks north of MTV headquarters in Manhattan, yesterday, Sony reps showed off three of the four big C’s featured in the upcoming PS3 first-person-shooter “Resistance 2“: Campaign, Co-op and Competitive multiplayer. (All but Competitive have mostly been kept from reporters until now, even though the game’s out in less than two months.)

I tried two of the three modes offered and learned a lot about how the most feature-rich first-person-shooter since “Halo 3” is shaping up.

First, I played co-op with seven other reporters and game developers. We played as humans in a mission against the invading alien Chimera, our squad of eight against a relentless horde. This mode is the one that developers at the game’s studio, Insomniac, claim is the most novel and distinct from other games in the genre. It is a bit different.

Insomniac reps told me that each level of the game’s co-op mode is set on three semi-randomly selected portions of the game’s environments, pulled from the competitive multiplayer maps. The game situates the players on one of six maps. Those maps have been divided by the Insomniac team into as many as 11 zones. The game randomly chooses three of the zones for players to fight through, configuring a set of basic combat objectives and ensuring that the third zone will be tuned to be the hardest — and probably contain a big boss.

Players can choose from one of three classes: a frontline, shield-wielding soldier; a mid-line medical-gun-shooting medic, or an ammo-resupplying and long-range combat special ops troop.

“We didn’t want it to feel like something you can beat by yourself.”

Jake Biegel, the design lead for co-op, told reporters that the mode can support up to 100 characters on screen at a time. A maximum of eight gamers can team hop. Biegel told me that you best not try this mode solo. You could try, but, he said, “We didn’t want it to feel like something you can beat by yourself.”

I asked why Insomniac chose three classes for the eight-player-maximum game, given that that would guarantee a tactically imbalanced squad. Biegel said that they decided that based on the desire to have three meaningfully complementary classes. The groups are balanced. The medic doesn’t have much offense, but he provides the only health power-ups available in the co-op sessions, courtesy of a healing gun. The spec-ops is needed for ammo reloads but only gains a bullet in his own gun for every ammo pack he or she lays out for the other players. Players of each class see a distinct tactical view of the game, with relevant data over fellow-player’s heads (e.g. the spec ops can see icons that point to whoever needs an ammo refill).

Every “Resistance 2″ player can be the same class, Biegel told me, but only if the players want a tough challenge. Medics, for example, may not initially carry guns, but they can be leveled up with experience points and ultimately be armed with some secondary offense. Sure, they could fight through. As long as an eight-man team of medics keep healing each other, they might be able to struggle to the end.

I felt diminutive, outnumbered and, ultimately victorious against the odds.

The feel of the level matched the developers’ pitch..Story in co-op will be told in modular fashion, through text pieces, according to Biegel. The basic narrative is that Blake, the officer giving single-player-mode hero Nathan Hale orders, commissions the eight-man co-op squad to go on a special mission to investigate advanced technology. This is also the narrative justification for the medic’s healing gun which otherwise does not appear in “Resistance 2.” Players who haven’t progressed into the later levels of co-op in the mode’s proper order will still be able to join friends who are playing in those later levels. Biegel explained that access to those late levels  is earned through a currency that you collect, and playing in the later levels with a friend will earn players extra currency so that they can go back and unlock the level for themselves and follow the story’s progression. Ideally, though, a player would progress through the levels in order.

I did not play competitive multiplayer at the event, but I was introduced, through co-op, to one of the key elements of that mode, Berserks. These are power-ups, like invisibility, creation of a healing radius, or enhanced ammunition that are temporarily triggered when a player has gained enough in-session experience points. Activating a Berserk is supposed to be a game changer. I didn’t survive long enough in co-op to understand their impact, but I can understand what kind of impact they would have.

I did play one and a half missions of “Resistance 2″’s campaign. I played through the first level, a 20-minute or so on-the-job training session in Iceland that occurs immediately after the end of the first “Resistance” game. I played follower to the aforementioned Blake, running through an Icelandic military installation while a skyscraper tall Chimera walking tank strode nearby. Insomniac is selling “R2″ as an FPS with dramatic scale. In this Iceland level, I felt diminutive, outnumbered and, ultimately victorious against the odds. The feel of the level matched the developers’ pitch. And that giant tank took a beating courtesy of some attack planes and a rocket launcher on Nathan Hale’s shoulder.

I was impressed with the progress of the three “Resistance 2″ C’s, but I was also left wondering if the fourth — community — was going to lag behind.

I played only a brief few minutes of a later “Resistance 2″ level set in a forest of California redwoods. I mostly watched others fight semi-invisible Predator-style enemies that kill the player in a single hit, wade through a river that ripples with a realistically-modeled wake, and face more waves of Chimera. No other set piece stood out. The level was action-packed and set in a memorable location but wasn’t punctuated with a water-cooler set-piece like the Iceland level was.

I was impressed with the progress of the three “Resistance 2″ C’s, but I was also left wondering if the fourth — community — was going to lag behind. Considering the fourth is expected to be just a website, how could it be nearly as feature-rich and relevant to the game as the other three Components? Details will remains mostly secret for now, but after a substantial but purely background conversation, Ryan Schneider, the longtime Insomniac PR man who is leading the project, had me convinced it will be something special. More details are coming on that element late next month.

While it once seemed that Sony’s “Killzone” series was the PlayStation brand’s answer to Microsoft’s “Halo” franchise, events like last night’s “Resistance” demonstration have made it clear that Insomniac’s franchise is the true challenger to Master Chief’s reputation. It has the scope, the scale, the design ambition and the ever-improving technology to merit a comparison. What to watch for is whether it can generate as complex a connected community of players and whether its elements will congeal to present the same quality of FPS combat from skirmish to skirmish as the best of Bungie’s “Halo” hits.