You haven't posted any screenshots of your arena. However, I suspect it is too small. If you have a large enough arena, Plantera should be easy to kill. You can use bombs or dynamite to dig out a large place for an arena (don't blow up Plantera's bulb!).
Also, you could put a wall or something in the middle of your arena and fly around it while shooting at Plantera. As long as you keep moving quickly, you should be fine. Make sure Plantera isn't leaving the underground jungle (even surface jungle is not OK), as she will enrage and pretty much tear your face off (she becomes orange-ish looking and her stats are doubled or tripled).
You should make a 1x2 pool of honey to dip in once in a while. Make sure you have potions, such as ironskin. For ammo, I recommend Chlorophyte bullets, as they are homing (and look awesome).
Note that you are using Turtle Armor, which specializes in melee. The Megashark (I think that's what you mean by Maxshark, because if you are using Minishark, then that's your problem) is a ranged weapon. You should try to obtain Shroomite armor. However, Shroomite armor has relatively lower defense. For melee, you should try to obtain a True Night's Edge, True Excalibur, or Terra Blade, if you are patient. Other than swords, I find Light Disks (as many as possible, I recommend 3+) to be helpful sometimes. The Death Sickle (from the Solar Eclipse, like the Broken Hero Sword) is extremely powerful against Plantera. However, Turtle Armor w/ Megashark is still an OK combination; you just won't get the melee buffs the Turtle Armor gives.
Since you appear to be dying too soon, try to get the Charm of Myths, which increases regeneration and decreases Potion Sickness length. From mimics, try to get a Star Cloak. Combine that with the Cross Necklace to get the Star Veil, which will be useful. Try to reforge your accessories to Warding, though if you insist on getting more damage (faster kill), try Menacing, which makes more than a little bit of a difference.
Avoid the Spike Balls (pink, have spikes) at all costs! They deal 100 damage (on PC, only 50). Other than that, you should constantly attack it without stop. Ranged weapons do work against Plantera, so your combination should be pretty good. Just make sure to make a big arena.
If that doesn't work, make a long narrow hallway w/ teleporters on each side, between 150 to 250 (NO MORE!) blocks long. Then, just run away from Plantera while shooting at her. This is pretty much a cheese method. Alternatively, you could set up a bed in the middle of your arena, and just respawn when you die.
Good luck and enjoy the fight!