Remember the old adage about 'bringing a knife to a gunfight', well I learned a better one while playing Killzone: Liberation - 'don't bring a rifle to a homing missile fight'. The odds are always stacked against you in this ruthlessly addictive game but, if you don't mind dying at the same point 17 times before finally scraping through to the next checkpoint, it's definitely a worthy addition to your handheld collection

Liberation is a very different game to the original Killzone on PlayStation 2, which is probably a good thing as most people don't have very fond memories of Sony's lacklustre Halo wannabe. The FPS action has been switched to an almost isometric third-person perspective that's more suited to the PSP's hardware and controls, while tactical play is rewarded over gung-ho blasting. Developer Guerrilla - also behind the original Killzone - has built this game from the ground up to craft a worthy action experience and, for the most part, it succeeds.

Liberation's plot picks up two months after Killzone, with that game's hero Jan Templar tasked with rescuing some goodie ISA hostages from the clutches of the nefarious General Metrac and his legions of Helghast soldiers. Oh, and you have to help liberate the key strategic area of Southern Vekta as well - but that shouldn't take five minutes. The first of the 16 missions sees you plunged straight into a landscape that wouldn't seem out of place in the trenches of the First World War, apart from the drop ships and laser mines that is. Enemies assail you from all directions and, compared to most intro levels out there, you really have to keep your wits about you to survive.

Unlike those top-down arcade classics like Commando and Ikari Warriors that Liberation superficially resembles, this is no straightforward blaster. Enemies can take almost a full clip of bullets before they die and, if you blunder around in a carefree manner, a lone bad guy is more than capable of finishing you off. Rudimentary tactics and a keen eye for finding cover are nearly as vital here as they are in the likes of Brothers in Arms or Full Spectrum Warrior. Fortunately, there are plenty of crates, boxes and bits of scenery to crouch behind, and you can pop out to fire at enemies by holding and releasing the right shoulder button. Clambering onto higher levels can also give you a handy vantage point, which proves vital when facing more than a single enemy. Learning when to fight, when to sneak and when to cower like a frightened animal is the key to surviving the tough as nails missions.

Killzone uses a helpful auto aim system to help you pick off the bad guys but, while it works in the most part, occasionally it can be a wee bit twitchy and unpredictable. This is forgivable in slower paced sections, but when the Helghast are coming thick and fast it can be frustrating to suddenly start targeting a random bit of rock in the background rather than the glowing yellow of their eyes. It doesn't help that your weapons are pretty weak. Even a sniper rifle takes three shots to polish off the average soldier and this can occasionally protract small firefights to ridiculous lengths. To hurry things along, you can always lob a grenade and watch the Helghast get hurled across the screen with their rag doll limbs flailing (I found shouting 'Die Helghast scum!' at such points helps too). There are also copious ammo and equipment packs littered around the levels and crates to smash that are filled with health and other items to collect, such as currency which can be used to buy weapon upgrades.

At points you are joined by another soldier, Rico, who can provide you with covering fire or just follow you as an extra gun to assist with major assaults. Through a simple menu system (which handily slows time whenever you access it), Rico can be ordered to target certain enemies or sent to cover depending on how you are approaching each battle scenario. It's a well-designed and implemented system that works superbly - so well in fact that you start to wonder why he doesn't join you for the entire game.

The visuals are gritty and among the best on the PSP

What most impresses about Killzone, aside from the solid gameplay, is the presentation. The graphics, while quite derivative of other shooters, are very detailed and the levels all have a real battle worn look about them that showcases just what can be achieved with the PSP. The voice acting and script are all up to scratch too, even if the plot is a bit limited and unengaging (the mission briefings are riddled with tiresome sci-fi battle clich├ęs). Although there is no online play (though this is apparently going to be addressed with an update), you can still enjoy up to six-player ad hoc Deathmatch, Capture the Flag and Assault modes, and the whole game can also be played through in co-op. Single-player Challenge Games (such as target ranges and kookier tests like catching scuttling spider mines in traps) further add to the game's pick up and play appeal. Just like those won in the main campaign, you can earn character points through these challenges to spend on essential weapon and kit upgrades to give you an advantage in the field.

With so much to praise, my only real criticism of Liberation is that it is brutally difficult. While the campaign is actually quite short (with only four levels), completing it requires an incredible amount of dedication and persistence that suggests the ramped up difficulty could perhaps be a cheap tool to pad out the length. Often you are hopelessly outnumbered and your enemies have vastly superior weapons to attack you with, like homing rocket launchers that can pick you off behind the apparently safest piece of cover. I'm a pretty hardcore shooter fan and I really struggled with some of the earlier missions, which was very dispiriting and could easily deter more casual players.

If it wasn't so mercilessly difficult then Killzone: Liberation would easily give current PSP benchmark Syphon Filter: Dark Mirror a run for its money. As it stands though, this is a slightly flawed but still impressive blaster that is well worth a look. Just don't be surprised when you keep seeing the same restart screen ad infinitum.