Try to get your team to stick to objectives and play asymmetrically. Relegate fighting to protecting/preventing the pre-made from reaching the objectives as much as possible, and don't offer any more targets for them to farm Battle High/Battle Fever.

Also, what I find best for myself is to heal against pre-mades. It may sound stupid, but IMO knowing how to heal in PvP can make a difference. You don't have to be a top tier healer to carry your alliance, you just have to be good enough to know how to mitigate bursts and keep your team alive - alongside with playing asymmetrically (i.e. focus objectives). Your team is going to get stomped on, there's no preventing that. However, it's possible to win the match, even if your team gets pummeled. lmao Basically, make it so your team "lose the battle, but win the war"... or "get destroyed, but still win the match". LOL

The other best option is to join a pre-made. O.o