Yeah... i forgot to mention that. You get the Stormbow by defeating Hallowed Mimics in the Hallowed. Its tuff. But once it happens, you MAY get the Stormbow, so Stormbow + Holy Arrows (Get a lot of them, it's arrows + pixie dust + unicorn horn) = R.I.P. Mechanical Bosses (Not so much Plantera though as it's underground.)
I Play On Terraria Mobile, So I Can't Craft Beenades. But Here's A Funny Story.
One Day In My Crimson World, I Was Going To Fight Queen Bee. I Summoned Her And Attacked Her With My Fiery GreatSword. When She Switched To Shooting Bees, I Used My Water Bolt. Little Did I Know There Was A Second Larvae In The Hive. I Was Like "OH MY GOD 2 QUEEN BEES? I DIDN'T KNOW THERE WERE 2 LARVAE!!!" After A Long Battle, I Defeated The Bees, Effectiveley Getting Me 29 Beenades.
The easiest is the destroyer, especially as a summoner. Then, you should fight the Twins, as their attacks can't go through walls (Unless dashing), which allows you to make a propler nurse spawn. Fighting skeletron at this point should be teasingly easy, since you can make items such as light disc and megashark.
The items I used was a nights edge and titanium armor. I used a clockwork assault rifle with meteor bullets to take down skeletron prime. For twins, you might wanna fight WoF until you get a ranger emblem, then do same as skeletron prime. For destroyer, it should be easy. Same strategy as twins, but this time stay in a sky box so you can notice and kill probes fast.
Get the slime mount and set up 2 target dummies. 8 blocks above the floor, place a block down. Use the slime mount and jump on the dummies. You should be infinitely jumping. Now, use a deadulius stormbow and summon a mech boss. Due to the 6 invicibilty ticks of the slime mount after jumping on an enemy (including dummies) and how you jump at a very fast rate, you are now invicible! Use the stormbow and fire at the mech boss.