Actually, I've been doing a Healer playthrough in the recent days, and I can give ya a precise feeling on the challenge:
the start is something hilariously bad, because you really can't find radiant weapons unless you find gold right off the bat and unholy shards from the blood moon enemies to craft the Heretic breaker, so you're stuck with Pills if you get lucky and find medicite and iron, I would suggest to make a suicide mission to the jungle and get all the materials required to make the Bountiful Harvest, the scythe version of the Blade of Grass; if you manage to craft it then your set for all the way to Skeletron.
Little advice: don't throw away the pills and make some Purified water, they're your only way to attack at range enemies, and you no wanna get near certain bosses; also make the other scythes to make the Falling Twilight/Blood Harvest, scythe equivalent of the Nigth's Edge along with a Crimson version as well, imagine making the Terra Blade process just with scythes.
all the non-flying bosses will be a nobrainer (the WoF battle will last about 10 seconds,30 if you take time to get rid of The Hungry), while the flying ones won't be difficult, they will just be time munching, especially the Star Scouter one since on the range side you won't have much DPS due to the slow fire rate.
The start of Hardmode will be the usual experience: can't kill, get smashed with the twich that thanks to the knockback and the DPS from your trusty scythe, you will have the power to defeat enemies
only one at the time: just try to not get ganked and you'll be safe, in case of mimics go ranged or just run; make sure to farm for hallowed enemies, you'll need their drops.
the mechanical bosses will require a bit of strategy: your DPS will be still low at the time as well as your defences, so you'll need an upgrade: make sure to make the Bone Baton,made with souls of night,bones(duh),and molten residue dropped by Underworld enemies, the Blood Transfusion, magic counterpart of the Vampire knives, the projectiles ain't very fast, but the heal-ability is just nice, the Null-zone staff which reduces projectile damage by 33%, really good angaist Destroyer, Prophecy, which you can cheese to get free health, and the Midnight Staff for some ranged extra damage+shadowflame debuff, try to get the Greedy Magnet too, so you can get all the hearts AND mana stars onscreen(statues are a must)
hallowed bars make really useful things for healers, make sure to get the Hallowed scythe and the Hallowed Blessing,with rispectively good damage and the ability to inflict a debuff that causes enemies to lose 3 hp per second and make them get 13% more damage; also try to get the Twinkle pet which creates healing ligths when hurt at the cost of some mana (amazing) I would suggest to defeat Destroyer multiple times so you can get all the Life Powered Cells required to make the Life Waver armor, with a mana regen so high you can cast low cost spells indefinitely when at max mana.
when defeated all three of the mecha dudes, look out for Solar Eclipses to get the 2 Broken Hero Schythes you need to make the True Scythes that you can combine to get the, you guessed it, Terra scythe, which has an AoE effect that NUKES things and makes all the bosses up to Golem kneel down at their knees. at that point you will be on my same point, just take it by the horns and smash those lads to the ground