I haven't seen this anywhere, but just some sources I put together:
When Tosfot Brachot 18b says “ שהוא חמור שבספרים” which we translated as “the most difficult” it could mean the most difficult on a spiritual/emotional level. Let me explain.
It is first important to give an intro as to why I refer to Vayikra as a whole as Korbanot:
The Ramban in his Hakdama to Vayikra says, והנה רוב הספר הזה בקרבנות בתורת הקרבן והמקריבין ובמקום שיתקרב בו. ויבואו בו קצת מצות נגררות עם אלה, meaning that most of this sefer deals with korbanot and then you have some mitzvot that get dragged along. And later on, he says, ובפרשת קדושים תהיו קצת מצות לבני ישראל רובן נגררות עם עניני הקרבנות או בדומים להם, that in Parshat Kedoshim there are some mitzvot that address Bnei Yisrael, but most of them are mentioned because of some connection with the offerings or matters similar to them. Therefore it seems like the entire theme of Vayikra is Kohanim and korbanot.
Next part:
The Ramban in Vayikra 1:9 (who disagrees with the Rambam) gives the whole explanation as to why we give korbanot. He explains that korbanot are a response to humanity's failures and aveirot. The stages of the sacrificial process are intricately designed to mirror the three facets of human actions through which aveirot are manifested: thoughts, words, and deeds. Actions that are aveirot are addressed through S'micha, where the person bringing the Korban places their hands on the animal before it's slaughtered. Aveirot committed through speech are acknowledged through vidui, the verbal confession made by each person. Lastly, the sinful thoughts and desires that have pervaded the person's mind and neshama are symbolized through the burning of the animal on the altar and the sprinkling of its blood. In other words according to the Ramban, a person should realiz and contemplate every time he brings a korban that it is he that should be on the altar burning. Because he sinned against Hashem with his guf and neshama. It says in Vayikra 1:2, אדם כי־יקריב מכם קרבן, so the Slonimer Rebbe says that the word מכם means, “Don’t simply bring an animal!” The korban should be מכם, from within you! Surrender your desires, and perspectives that arise from your animalistic soul! Only when the sacrifice comes from within you does it become a "korban laHashem.” Then you become close to Hashem. This is a hard thing to do and takes a lot of strength spiritually speaking to get there!
Furthermore, the Rebbe in Likutei Sichot first volume Parashat Vayikra page 196 the Rebbe explains what korabnot are in the spiritual sense, that a korban, after there is a fixing of blemishes, needs to be shechitah of the body and animalistic soul, meaning, there needs to be an exiting of the live spirit of the animalistic/physical desires. The physical stuff stays complete but it's not living/no spirit anymore to it, and is it now only used for extra strength in avodat Hashem as it says in Yoma 76b (and many more examples like this) where Rava says חמרא וריחני — פקחין, wine and good scents make me wise, meaning that if one uses the physical pleasures for the right reasons (avodat hashem), one becomes wise and stronger in his avodat Hashem. The Rambam in Hilchot Deot in the third chapter and Tur/Shulchan Aruch O.C. in Siman 231 say that a person should use the pleasures of this world leshem shamayim, for avodat Hashem. The Rebbe adds that it's not using the pleasures of this world for holiness, but actually turning the pleasures themselves into חפצא של מצוה. So also based on the Rebbe, we can say that it is difficult and takes a lot of strength to take physical desires and turn them into our avodat Hashem because its against our nature to do such (it's a miracle that the body and soul can function together to do Hashem’s will since they are complete opposites).
Therefore I think that the lion was the animal the Gemera uses because a lion represents strength. This is seen in many places, including Pirkei Avot 5:20, יהודה בן תימא אומר, הוי עז כנמר, וקל כנשר, ורץ כצבי, וגבור כארי, לעשות רצון אביך שבשמים, meaning strong like a lion to to the will of Hashem. Also in Vezot Habracha it says 33:22, ולדן אמר דן גור אריה יזנק מן־הבשן, and many Rishonim explain like the Daat zekeinim that it means, שהיה עומד על הספר ושוקד ושומר על ישראל שלא יכנסו (אויבים) לארץ, that Dan’s soldiers were constantly guarding the border so the enemies don't attack them like lions. This can be compared spiritually speaking to constantly being on guard on the yetzer hara from us to do aveirot, which takes strength just like lions.
This maybe explains what it says in Brachot 3a, ועל כל משמר ומשמר יושב הקדוש ברוך הוא ושואג כארי that Hashem roars like a lion. Why is Hashem compared here to roaring like a lion over the beit hamikdash? Perhaps the explanation is that since the beit hamikdash was all about the lion mentality, Hashem misses this mentality of bringing korbanot which represents the strength and difficulty of according to the Ramban surrendering ones desires, and perspectives that arise from their animalistic soul, like sacrificing a part of themselves. Also according to the Rebbe, about subjugation of our physical pleasures and even turning them into ways to serve Hashem even though physical and spiritual are not supposed to go hand in hand.
Here is the likutei sichot:
Link used: https://torah.org/torah-portion/parsha-insights-5757-vayikra-2/
