To start things off, Tsuna have the AP advantage by an unknow amount.
Mega Absol have a very decent variety on his movepool, hitting hard with STAB Dark Pulse and Night Slash, Pursuit and Mean Look for the hits and runners, and a good amount of Stats reducement in the form of Snarl, Zen Headbutt, Megahorn and Thunder are also very good moves when combined with his IQ skill that raises his accuracy and Future Sight to attack in a blind spot, while having Psycho Cut and Play Rough for CQC. Absol have power null in the form of Taunt, there is also Leer to just screw up with Tsuna defenses, Swords Dance to amp Absol's attacks, three forms to temporally amp his speed, with Feint, Quick Attack and Sucker Punch and a good Precog in form of Detect. Absol passive Fear Manip that is boosted by his IQ Skill Intimidator are also really deadly
With all of that saided, Tsuna can counter alot of things there, he has resistance to power null, so Taunt won't work, while forcefield can be bypassed with Feint, that is too weak to really do significant damage to Tsuna, he also has his own Stats amps, but that can be punished with Punishment. His Precog, while not that strong, could help him in evading Future Sight and Moves with high priority. Tsuna can also fly to help him in evading incoming attacks, Mean Look and Pursuit are good moves to prevent that, but he can still fly even with Mean Look active, and Absol won't use Mean Look at the start of the battle, since he hates fighting. As i saided, Tsuna has his own Stats amps, so that can counter Absol's ones, Tsuna also have goods moves with his Ice and Fire Manipulation, while not really effective against Absol, they should give him some trouble, especially his Ice one. Tsuna has his own Power Nulls, and thei problaby are more effective than Taunt, since he can Nullify every type of Attacks, meaning that he can prob nullify Thunder and other range attacks from Absol, but the thing that Tsuna really have on his edge is Petrification, that can guarantee a clear win for Tsuna.
The thing that Absol really have here is Fear Manip, Range and variety, the Fear can really affect Tsuna, since he is in the end of the day, just a human, and that Fear can prob affect Natsu, but i think in a less extent, his Fear Manip could even make his dodging less effective.
Overall, this is a much more Close fight than I imagined, Tsuna have the AP and alot of abilities that can help, but, for me, Absol's Passive Fear Manip will really play a alot here, giving him the win with High-diff