I usually get a fair amount of kills with the mantis, and this is my best advice: Don’t get greedy, and don’t play aggressively.
As a mantis pilot, you basically have the advantage against any individual opponent, so let them come to you. You’re not out to win the game single-handedly, the vehicle is all about area denial and defending the home turf. You’re going to be a support and defense player. Think of yourself more as a mobile defense turret. Stay on the friendly side of the map and keep to areas where your team mates will spawn behind you; this means you don’t have to worry nearly as much about attacks from behind.
Use the terrain to your advantage and keep some line of sight blockers between you and long-range threats; mantis is highly vulnerable to long-range weapons like snipers and splasers, and can’t effectively counter anything outside of BR red reticule range. The mantis is terribly ineffective at longer ranges due to bullet spread and slow-moving rockets. If you’re positioned right, your enemies will be forced to fight you on your terms, in closer ranges. Keep an eye out for people peeking over hills to potshot you; you won’t be good at countering this, your best response is to pepper them with chaingun bursts to scare them while you fall back to more cover.
Cover is extremely important. If you can duck behind cover when your shields run down, you can wait for them to recharge, preventing physical damage to the vehicle. Play smart and use this to your advantage, you’re in the only vehicle with shields, with this tactic you can shrug off critical damage near indefinitely.
Keep your chaingun cool and your rocket count full. Chaingun works great to pepper enemies at mid-range while rockets should be reserved as a followup punch and for closer encounters. Your weapons don’t amount to much at longer ranges so avoid these scenarios. If you do have to engage at longer ranges, fire a rocket to the left and right of the enemy so he strafes into the fire, and tap your chaingun once every two or so seconds; it’s only range accurate for the first couple bullets out of a cold barrel.
Don’t rely on your stomp, as the enemy will be expecting that. If they get close to you on foot, take them out with rockets, keep maneuvering and facing them with your front side.
It’s not about being the highest scoring player, it’s about staying alive as long as you can, so you can keep threatening the enemy and prevent them from flanking your team.