Are Lutherans the "adversaries" of Catholics, according to the Catholic Church?

Nothing prevents the baptized from becoming heretics and thus opposing the Church. Protecting the Church against harmful influence ("adversaries"), like the arch-heretic Luther, is the rationale for trying heretics and, if necessary, even delivering them to the State for punishment (e.g., the death penalty):

St. Thomas Aquinas's Summa Theologica II-II q. 11 a. 3 c.:

With regard to heretics two points must be observed: one, on their own side; the other, on the side of the Church. On their own side there is the sin, whereby they deserve not only to be separated from the Church by excommunication, but also to be severed from the world by death. For it is a much graver matter to corrupt the faith which quickens the soul, than to forge money, which supports temporal life. Wherefore if forgers of money and other evil-doers are forthwith condemned to death by the secular authority, much more reason is there for heretics, as soon as they are convicted of heresy, to be not only excommunicated but even put to death.

However, he advocates mercy; he continues:

On the part of the Church, however, there is mercy which looks to the conversion of the wanderer, wherefore she condemns not at once, but "after the first and second admonition," as the Apostle directs: after that, if he is yet stubborn, the Church no longer hoping for his conversion, looks to the salvation of others, by excommunicating him and separating him from the Church, and furthermore delivers him to the secular tribunal to be exterminated thereby from the world by death [as a last resort]. For Jerome commenting on Gal. 5:9, "A little leaven," says: "Cut off the decayed flesh, expel the mangy sheep from the fold, lest the whole house, the whole paste, the whole body, the whole flock, burn, perish, rot, die. Arius was but one spark in Alexandria, but as that spark was not at once put out, the whole earth was laid waste by its flame."

Thus, St. Thomas does not indiscriminately advocate the death penalty for heretics, but this illustrates heretics' danger to the Church; thus, heretics are adversaries.

When one asks what any church body believes, one has to go to their authoritative sources. For Roman Catholics, one of those authoritative sources is the Council of Trent. In session 6, canon XII, they write (https://history.hanover.edu/texts/trent/ct06.html):

CANON XII.-If any one saith, that justifying faith is nothing else but confidence in the divine mercy which remits sins for Christ's sake; or, that this confidence alone is that whereby we are justified; let him be anathema.

This condemnation was included (along with a number of others) to clearly and specifically speak against what Lutherans taught. And since what Lutherans taught was against the official teaching of the Roman Catholic Church (as approved by a pope and a council), all Lutherans are now "extra ecclesia" outside of the church (i.e. destined for hell). This is the official teaching of the Roman Catholic Church. And they have not changed these words or repudiated them.

The second distinction one has to make with the Roman Catholic Church is that they, within themselves, make a distinction between what a pope might say on his own and what he pronounces "ex cathedra", occupying his office as the vicar of Christ, officially making a proclamation on behalf of Christ.

This is important to understand because it is not uncommon throughout history to see an odd variance (even contradiction) in the teachings/preachings of the bishop of Rome. Please allow 2 examples.

1. In 2007 Pope Benedict XVI reminded the press that all churches that weren't within the sphere of Roman Catholicism "were not true churches." (https://www.msnbc.msn.com/id/19692094 ) (https://drive.google.com/file/d/1bfb9sCrKA9eH3loRC9XixVUBPGbuDiHj/view?usp=sharing).
2. In 2013, Pope Francis states that atheists can get into heaven (https://www.npr.org/sections/parallels/2013/05/29/187009384/Pope-Francis-Even-Atheists-Can-Be-Redeemed )

How can it be that one who believes in Jesus (who has not submitted to papal authority) can go to hell and yet one who does not believe in Jesus at all can go to heaven?

That tension, for Roman Catholics, is resolved by the pope making pronouncements "ex cathedra" (officially as the vicar of Christ) and also by councils gathering together to make commendations and condemnations.

The condemnations against Lutheranism have never been revised or repudiated by a church council or by a pope speaking 'ex cathedra'. Lutherans, because they believe that they are saved by Jesus alone through faith alone are eternally condemned (anathema sit) in the eyes of the Roman Catholic Church:

Canon XII -- Si quis dixerit, fidem justificantem nihil alind esse, quam fiduciam divinae misericordiae peccata remittentis propter Christum; vel eam fiduciam solam esse, qua justificamur: anathema sit.

Protestants are called so because they started to protest against practice of the Vatican in the 16th century. The Vatican refused and banned the protest. This led to churches being adversaries.

However, that is history now. In practice, the Catholic Church even has dialogues with Islam. Considering Lutherans as adversaries is definitely outdated.

Bellarmine wrote arguing the Catholic position and attempted to demonstrate that the Lutheran position, on a range of matters, was incorrect. He frequently did this along the lines of "our adversaries argument is this, but I think they are mistaken because..".

In that contest "adversaries" means those on the other side of the question, those putting an alternative opinion.

He was not saying he regarded Lutherans as his enemies. There were no unfriendly, pejorative, hostile or even impolite intent in referring to a person on the other side of a debate as an adversary.