The achievements do save across all four of the files on the computer on which you're playing Sonny 2 (this is actually necessary for the All-Star acheivement, as you have to do one Legend-optional Heroic run with each class to get it).  The achievements also save automatically the second you get them even if Autosave is turned off (something I found out when I got Black Magic on a run that I'm still working on through this guide).  If you save over an existing file you would still have record of any achivements obtained on the previous file.  Also, if for whatever reason you need to clear the cookies and cache on your browser the data will still be there on your computer because the data's saved in the Flash Player's files independently of the browser.

There's a few battles in zone 2 and most of zone 5 in which I would consider difficult.

There is 1 hard one in Z2 which is Captain Hunt and in Zone 5 i didn't really struglle with any of them, on 1 or 2 battles i had to re do it because a team mate went out very early on, tbh i found the Colonel probably the hardest in the Zone as i died to him 2 times before completing.

6: (Level 19) - Care Taker, Blood Hound (2)

I need help <_<.

Why does everyone struggle with that level? The Cold Hydraulic is tbh the best suited for it. I did it on my first try. Anyway Vicxyz check pages 3 onwards because Techno was stuck on it aswell so there is a lot of helpful advice on there

I'm on the one before Level 21 though. Is the strategy the same? Freeze Combo Specialist then Hounds?

Excuse me, Care Taker*

Nevermind. Somehow before I wasn't even able to take down one of the enemies... it was fairly easy surprisingly...

It seemed to be bad luck on my part primarily (though the fact that Vera was short on XP due to me going for Black Magic didn't help either since he was just short of 21), but yeah, the strategy should be more or less the same, though I ended up Freeze Comboing one of the Hounds first, repeating with the other and finishing with the Specialist.  I also had to equip the Mining Helmets and items that granted Vitality and Physical Defense bonuses.  It's weird because I wasn't having as much trouble with the one before it...

Ah well, now I'm struggling with the Guardians of Z7 (getting better, but for some reason the weaker one still dies before I can get the other down to the same level and I end up using Ice Wall to delay the inevitable until my party is thoroughly decimated by the survivior's Lightning Bolt O' Death).

7: (Level 19) - Android Guard, Riot Police (2)

Is it easier to attack the Android Guard first?

I just ignore the Android until the Riot Police are gone.  It seems hard to ignore it at first with its debuffs and everything, but unless he casts a shield buff on one of the RPs you can safely ignore it.  The Android wastes every fifth turn recasting its shield, and when it gets low on Focus it goes into a self-inflicted stun for about 4 turns.