Be patient and slowly ware down their life. There arent any real good strategies to some of the bosses but try different combinations if u r having trouble passing a lvl. Start on easy mode to get the jist of the game and work your way up as your sonny 2 gaming experience increases.
If your Hydro then I think Frost spec is best - If not just the starter move Vapour Cannon can wipe the Fire Claws - The hydra himself is easy with Veraduax as Phalanx / Defensive and Roald as relentless
For the Baron: If you're a Bio just set your team mates on defence mode and keep using dirupt on Brixious (if that's how you spell it =/) and wjen disrupt is recharging just keep you team mates defending and let Sonny use all his best stuff on the Baron and he will die eventully.
For the Hydra: I just used infect for the one on the far left and envenom on the one on the far right and then keep using destroy and other brutal strength related attacks and then te middle one will eventully die. NOTE: Keep your team members on neutral for the Hydra.
For the mayor: Just use loads of strikes on the cannon until the sheild goes and destroy thr cannon and then use all your moves on the mayor and he will go quite easily.
The mayor is very easy, just spam lots of attacks on the cannon, 3000 damage isn't a lot of shielding, in about 2-3 turns it should be down, and also it uses vapor cannon, the water vapors effects somehow still counts as a debuff for shatter if you're hydro, then spam everything on the mayor, he doesn't do too much damage, even with enrage