Glad to see a new face taking interest in Lockdown! But you seem to have some misconceptions regarding usefulness in PvP. Assuming Striker is the only viable option and that the other two classes are worthless, is an extreme overestimation of striker capabilities, and underestimation of the other classes. Since Lockdown displays the damage stat at the end, naturally people are going to want to have the highest number. This leads them to pick Striker as its mobility enables this high number to become a reality. The other two classes have their own uses, which I will touch on later.

PvP mentality is completely different than the mindset you use in PvE. Survival should not be a main focus, as you will always respawn and each class has a defensive option to fend off attackers. Your goal is either to capture/uncapture points, attack enemies, or both. For this you will first need gear. Some gear thats amazing in PvE (i.e Blitz Needle, Graviton Vortex) have very limited uses in PvP.

Common swords are those of the Flourish/Snarble Varb line, renowned for their speed and range, the Sealed Sword line known for its slow but devastating attacks with substantial range, the Brandish line which combines power and speed but sacrifices range, and of course the Acheron, which you can group anywhere really. The best guns are from the fast firing Antigua line, the slow but heavy Magnus line, the somewhere-in-the-middle Blaster line, the mindgame nightmare Alchemer line, and the walling Pulsar line.

Bombs are heavily underused, but can still retain use as area denial tools. For mist bombs, once your armor reaches a certain amount of status resistance, they will be unaffected by travelling through it, referred to as status walking. For example, a full Skolver can walk through a Shivermist Buster and not get frozen. Due to the most powerful armor sets containing base Freeze Resistance, this isn't a very popular pick. Stagger Storm and Voltaic Tempest can be devastating to walk through, but some players have special loadouts meant to walk through these as well. Venom Veiler and Ash of Agni are not normally defended against, but that's a result of the status not affecting their movement, and thus their ability to be killed. The two status inflicting shard bombs, Scintillating Sun Shards and the Shocking Salt Bomb can be tricky to use and land, but can still be effective. The damage of Dark Briar Barrage can still be deadly, and the knockback of Nitronome is something that will keep players on their feet.

For armor, any set that doesn't include stat bonuses is practically worthless. These attack speed increases and damage increases are vital for competition. The Wolver line can branch into Skolver/Snarbolax and Vog Cub, which grant sword damage and speed increase respectively. Similarly, the Gunslinger set can utilize the Shadowsun set for damage, and the Justifier/Nameless set for attack speed. Bombers can use their own demo set, with Volcanic Demo for charge time reduction, and Bombastic Demo for damage, or Mad Bomber for both, but most bombers just use Chaos, as it provides the same bonuses as Mad Bomber (except universal) with the same defense and resistance (except with curse weakness). Then there is Chaos and Black Kat, the former of which provides a damage boost and charge time reduction for all weapons, and the latter providing high damage and movement speed increase. Black Kat is harder to come by though, since it is event only.

When making PvP gear, heating and upgrading is a pivotal part. While you might want to upgrade your Swift Flourish into a Final Flourish for more damage, do not proceed unless you have enough Radiant Fire Crystals to heat it to Level 6-7. If the five star weapon is a lower heat level than this, it is actually a damage downgrade (unless we're talking about bombs, in which case, ignore this rule). Ignore this rule for armor as well, and upgrade as soon as possible to reap the bonuses that five star armor gives you. For trinkets, if you're playing striker, two Penta-Heart Pendants are a must as they give you more health and wont make you shatter like glass when hit. For Recon and Guardian, you can use health if you like, but the other stat boosting trinkets can help give you an edge as well. Of course, shields do nothing in Lockdown, so ignore those.

So once you're geared for PvP, enter Lockdown and practice your combat. You will be fighting several players over these points after all. Mastering shield cancelling, charge holding, not firing full gun clips, and other advanced techniques will be crucial for your success. Once you have a loadout you are comfortable with, go on to capturing points and attempting to win. Sometimes winning doesn't involve just capturing points, sometimes just uncapturing a point and preventing the enemy from gaining points can lead to your victory. Consider those as alternate win conditions, even though the game will not recognize them on the results screen. If you plan on going after points, don't always head for the point nearest to spawn. If most of the enemy team is close to your base, going around them and uncapping/capping a remote point (known as backcapping), can distract several members of the enemy team and level the playing field for the match.

There's still alot more to go over, and more information to process. But this knowledge should help jumpstart your Lockdown endeavors.

Good luck~