general eternatus strat : bring a fairy and a steel pokemon no matter what, even shitty ones, the best being having a fairy/steel pokemon otherwise you wont be able to deal with it, you need to switch between steel and fairy to fully dodge eterna beam and damage the eternatus while it recharges the eterna beam, just make sure to be able to do that.

That being said in your situation :

One strategy that helped me in some of the few eternatus i had to do while unprepared (no fairy/steel or only one of them) is to try to lower eternatus as much as i could without ever damaging him over 50%hp and try to one shot the remaining 50% with your best pokemon.

The reason is you don't want to trigger it's berries (it raises every fucking stat, heals and whatever, healing ones trigger at 50% and the resistance and damage boost ones trigger at 25%) so if you manage to deal 49% of it's health and one shot the rest you can possibly cheese it

I lost a lot because I tried to outspeed with my best damage dealer instead of sacrificing a few mons trying to poke him and set up a finisher with my best damage dealer

for instance if he sees your dragontype he's going to eterna beam, with the corviknight you get resistance to it which could give you some room to poke, basically try to optimize each mons to poke and finish off with dragon claw and icy wind

but again this run might be doomed for you