It was five Deviants total in the main jungle fight. Also, look at how out of shape professional athletes can get with just a year or two away from their sport. Now think about these Eternals not having to fight anything for over 400 years. They may not have been physically out of shape because their synthetic bodies don't do that, but their mental skills and fighting skills and teamwork skills turned to mush in that time. And also, when they finally get back together to deal with the thawed out Deviants, they have to do it without the one who was their leader for 7000 years. And with a traitor trying to screw them over.
That is an explanation, sure. But it's not well supported by the movie. Based on the movie they walked off their spaceship at apparently peak fighting capability despite "never" fighting these deviants before. Which do you think is worse, coming back to fight something you've fought for thousands of years, or fighting something for the first time?
In fairness though, the overall scope and extent of the fighting they had to do over those thousands of years wasn't ever really established. Was it a few deviants a week, a decade, a millennium? It may be that the Eternals are less a crack team of hardened warriors and more like reservists in the national guard, called up for emergencies and some occasional training exercises but otherwise doing something else with their time.
Ultimately, it was one of those things where I kept asking myself, even during the movie, "How powerful are these characters supposed to be?"
TBH, I still don't know the answer, so the best I can come up with is "as arbitrarily strong or weak as the plot requires them to be."