Telepathic battles are known for getting weird; after all, anything is possible with the ability to enter someone else's mind and warp their imagination. But, the weirdness from Professor Charles Xavier's first telepathic battle actually derives from the "battle" part rather than the telepathic. Believe it or not, New X-Men #121 revealed that Professor X's first battle was actually in his mother's womb.
Now to understand this fight, one must be aware of Charle Xavier's twin sister Cassandra Nova and what exactly she is. Cassandra is known to the Shi'ar as a Mummudrai. The Shi'ar are an intergalactic empire whose religious beliefs cause them to traverse the universe forcing different species to marry them in an effort to strengthen the race through evolution. A Mummudrai, according to Shi'ar myth, is a spirit that is equal and opposite to another person's being. Essentially, a Mummudrai is the anti-self in a spiritual form that must be defeated before one's birth. Due to the incredible power of Professor X, his birth resulted in a Mummudrai so powerful that it actually went beyond spiritual form into a physical self, resulting in Cassandra Nova.
So how exactly did this battle go down? New X-Men #121 kicks off with Professor X lying in a mental prison, trapped telepathic in the mind of a comatose Cassandra Nova. Jean Grey and Emma Frost, being the top telepathic X-Men aside from the Professor himself, use their abilities to go inside Charles's mind in an attempt to rescue him. It is here, inside this mental realm, that Jean sees the memory of Charles in the womb. After a bit of exploration and a few hiccups, Jean and Emma eventually find the Professor in this mental realm, and Jean dives even deeper.
While here, she witnesses the memory of Charles's very first battle. While in the womb, his power was so great that his Mummudrai took physical form as a twin sister within the womb with him. Cassandra awakens within the womb, and, using his own umbilical cord as a weapon against him, begins to strangle Charles. Charles, struggling to survive, lets out a massive psionic blast. The blast ultimately results in the mother falling down the stairs, causing a miscarriage leading to Cassandra being born as a stillbirth. That's right, the moral center of the X-Men once threw his own mother down the stairs to kill his own sister.
Despite being a stillbirth, Cassandra clearly lives on, which is what eventually leads to this mental confrontation, to begin with, but luckily Charles has clearly beaten her before. It seems even before birth, Professor X is definitely not a mutant to mess with - New X-Men #121 weirdly proves that.