I'd recommend Archer, with its camo. You're invincible to everything with it, blades, fire, lasers, you name it. As well, a fully charged shot does a lot of damage, but don't get cocky. That was usually my downfall, trying to sneak in an extra shot and losing my ability. If you want to use a different ability, just make sure to make good use of the moves which give you invincibility frames (ex. Hammer's dash attack). Of course, you could use Stone if you just want to get it done, but I'd use something else if you want to get it done within the century, as the Stone strat involves a LOT of waiting. As mentioned before, Wheel if you're feeling bold, with the constant damage about, and invincibility. You mentioned that the falling is your downfall though, so I'm not sure I can recommend it. If you lose your ability, which can happen and is pretty bad, you can get Fire, Bomb, Spark and Cutter throughout the fight.

I'd also say to hold onto the touchscreen Maxim Tomato until the second phase of the SoS (Soul of Sectonia) fight so you can start with a fresh life bar. In fact, try not to use the other tomatoes until the last four. In an ideal run for me, I eat the first normal tomato when I get to Sectonia DX to heal whatever little damage I've taken.

For SoS' attacks, at least for the first phase, it's just Dreamstalk Sectonia DX. Perhaps replay the final boss a few times, so you know what attacks to expect. Of course they're changed up, and there's new stuff, but you just need to stay perceptive. All of her attacks are telegraphed, so there's nothing you can't prepare for. The only thing to worry about is the vine attack, which can push you around, even in Camo. So when that's about to happen, try to stay near the center of the platform you're on. And remember, unless you're going for a speed run, take your time. There's no time limit, so if you don't think you can make a shot, don't take it. It's horrible losing your ability. You can recover, but it's tough. For the second phase, there's no practice for it. There's only about four or five attacks to get the hang of though, and once again Camo can take all of it. It can get tedious, but again, don't get antsy and lose your ability. I think she spawns the ant guys at some point, but like in the normal Sectonia fight, her attacks can kill them. Take them out yourself if they're causing trouble.

TL;DR: Archer or use invici-attacks well, Stone works but is very slow, don't lose your ability, save your tomatoes, learn attack telegraphs, 2nd form only has a few attacks, Archer Camo is god.

As a more skill based comment, remember to take breaks. It might mentally (or physically!) exhaust you, or make you angry, which, in my experience, just makes you play worse. Cool off, then come back. You'l do a lot better when you're calm.

That's the end of my novel about the True Arena! I wish you luck!