Terraria – the easiest way to get up to a floating island

From the Terraria wiki page on Trees:

Growing conditions

Planting an acorn in grass will create a Sapling that will eventually grow into a tree of a random size and shape. Saplings can only grow under the following conditions:

1. A planted acorn can only be planted on a dirt block covered with grass, corrupt grass, crimson grass, hallow grass, a mud block covered with jungle grass, or on snow.
2. The acorn requires one space of open ground to either side.
3. There must be at least two open tiles between saplings or only one will grow into a tree. This matches the previous rule, i.e. each acorn needs both adjacent tiles free, which means the closest two trees can be is with two tiles between them.
4. The acorn requires at least 32 feet (i.e. 16 tiles) of vertical clearance to grow into a tree.
5. An acorn will not grow indoors - all background walls or overhanging ledges must be removed, and if you have a ceiling it must be at least 32 feet above the block where the acorn was planted as measured by a Depth Meter (e.g, if you are planting an acorn at 300 feet above, there may only be blocks at 332 feet or higher directly above it in order for it to grow into a tree).
6. Acorns will not grow in the presence of torches, wood platforms or sunflowers within its 2-block radius.
7. Saplings must be off-screen in order to become trees. They will not grow when currently in view, unlike most other plants which can spawn while you watch.

Saplings can be destroyed with a Pickaxe.

So, you can't grow a tree at the edge of a step or cliff, or next to a step or cliff, or at the edge of a patch of grass, even though you can plant acorns there. While trees can, very rarely, grow a distance of three tiles apart (two empty columns between them), they grow much faster at a distance of four tiles apart (with three empty spaces between them). For these reasons, it is recommended to plant acorns every four tiles on a long flat row of surface grass. Tiles more than 14 units above the planted grass do not have any effect on tree growth, so an optimal tree farm can have multiple 15-tile high levels."

You seem to have your acorns spaced correctly, there are no walls, and I assume you have enough head room. Although it's not explicitly mentioned in the wiki, your torches may be keeping the trees from growing. Try removing them. At least some of the acorns should grow to full trees in a matter of minutes.

I'd say go for it.

You might need a decent amount of health potions, but you have enough maximum health and a good armor to withstand his attacks. There's really not a lot you can do to dodge his attacks without Rocket Boots, especially the arms.

Obviously, ranged weapon are best. I recommend the Demon Bow and the best arrows you have. As a melee backup weapon, something with a very long reach is preferable (e.g., the Phaseblade or the Blade of Grass), but you should really pack enough arrows to make that unnecessary.

If you decide to brave the Underworld before doing battle with him, replace the Demon Bow with the Molten Fury and add the Flamarang as a backup weapon. I did so too, but I found surviving in the Underworld long enough to get a decent amount of Hellstone a lot more difficult than the Skeletron encounter. After the Dungeon, it might be more manageable if you found a Blue Moon.

Although, risking your hide just long enough to gather the 45 Hellstone required for the Molten Fury might just be worth it, since it does a lot of damage — more than double of what the Demon Bow does. Plus, you won't have to waste any torches on Flaming Arrow crafting.