Cassandra having a chance against them? Right now, no... The power gap is way too high. When Cassandra reaches their normal duel range, I believe she'll have a chance to defeat them, or at least put a hell of a fight. I've been in your shoes, ( the level gap was smaller, like 150 levels of so), I also thought " how the hell am I going to be able to beat this guy?", I couldn't see myself getting strong enough to be able to do so...
I analyzed things coldly, subplasma attacks weren't doing any harm, so they could be safely ignored... They were a sort off wake up call, making me realize this was indeed a pvp game( guess this makes manunkind my first, lol)... The only thing I didn't like was the torture chamber, so I made it a priority to get the scarab of death ( I was also getting close to Vesco's level and he was scary as hell

)... I end up setting my goal on being powerful enough to defeat the strong doomlords that crossed my path as I leveled up ( also making my best to maximize soul energy to experienc ratio and skill/ ability cost reduction)... There is always some sort of treat on the horizon, some new goal to achieve... And even after all this time I still feel the urge to improve fast ( a.k.a. fear) when I realize there is someone I can't beat ( and my teacher still teases me for it

, perfectly justified).
I know it seems impossible now, but trust the process, improve your character and the results will start to appear... Will it take a few years to reach Manunkind's duel range? Absolutely. Will be attack them with subplasma? Probably yes. Can you, from a gammer's perspective ignore those attacks? Yes you can... The part of if you want to ignore them or not, I can't answer... Each one of us needs to make that decision, I made mine years ago, and I'm glad with the results... For now!
Ps: as for level differences... I'm level 267, my clan leader is level 400 ( 9) when we battle in the clan arena sometimes I lose, sometimes I win... Level isn't everything , but it sure helps.