Even at a young age, Xanxus was able to channel his sky flame into his hands. His ability to channel the flames into his hands also transfers to transferring the flames into his guns. While using his flames he gains great speed and strength. In some cases, he even uses the flames as a means of propulsion. Xanxus also has a super intuition that allows him to quickly understand his opponents; it acts as a sort of sixth sense which usually allows him to see things that others can't. It is just shy of being as good as a true Vongola's. He is stubborn and refuses to let his men heal him because he believes he's not so weak as to need it. Most of his abilities stem from his ability to use his flames of wrath as is the way of the KHR world in which each person has a particular flame (something akin to auras or energies). This flame is not only his claim to strength, but his means of keeping power. Since he has nothing to lose, he fights with every fiber of his being when he decides he's going to fight. He may screw around with his enemies but few rarely leave the battlefield alive.
His main means of fighting is through the use of his flames (with or without his guns). In the Future Arc, he also has a box weapon, but he's so cool that he gets his own section. The basic breakdown however, is that Xanxus' rage fuels his flames. With that rage he is able to channel stronger and more dangerous power. He is rarely without energy and injury in a meaningful way. He does get injured, but through sheer force of will he heals and keeps moving forward. A particular strength that I find in him is that despite all odds, he keeps himself portrayed as the embodiment of how he defines strength. He always holds himself to the challenge of his ideal strength. He never wavers in his resolve, his decisions, or himself. Something that for all his negativity, is quite charming to me. In fact, the one thing I can say is that I appreciate his confidence.