I've been playing summoner, pre-plantera, wearing Spider Armor, Summoner Emblem (with +4% damage modifier), Pygmy Necklace, and Cool Whip.
I ground on as long as I could with the Spider Staff, even making enormous background walls at my surface arenas for the spiders to climb on, but it wasn't working. I had to switch to Melee-hybrid for boss fights: the bananarang and frost armor worked pretty well. I used the power-glove instead of the Summoner Emblem but kept 2-3 minions and a couple sentries from the Old Man's Army, so I still felt summon-ish. I was able to defeat The Twins this way.
The Dreadnautilus was hard to fish up, but pretty easy to defeat and I got the Sanguine Staff on the first try. The bats are SO MUCH BETTER THAN THE SPIDERS! My damage-per-second was like double or maybe 4x what I got with spiders.
I used a blood tear to summon the blood moon, then fished from maybe 12 blocks above water in a house in an arena with platforms, using the Chum Caster (higher chance to catch blood moon bosses) and just about every fishing buff item and potion (even the toilet, sake, and beer). Pretty sure I used just the Inferno potion for the fight and the Nautilus stayed right in the ring of fire. I think I caught the Nautilus at 3:55AM and the fight was done by 4:15, so about 20 seconds. The hard part is all the other blood moon stuff and switching from the fishing setup (hot-key F3) to fighting (hot-key F1), and downing the inferno potion without getting killed. Because I used melee and a 2nd Pygmy Necklace for my fishing setup, I didn't have to re-summon anything.
I upgraded to the Durendal and Blade Staff at the same time, so I can't say which made the bigger difference, but it was definitely a step-up. Once I had them, I didn't need the frost-armor/bananarang/glove any more. I think I had to fight Queen Slime 7 or 8 times before she dropped the staff. That was rough, but doable with 6 sanguine bats and 2 sentries in a good arena with a honey puddle, heart statue, heart lantern, campfire, etc.
Once I had the Durendal and Blade Staff and added a few Honey Bubbles to my main arena, I was able to defeat The Destroyer with the full Summoner setup and every relevant potion (except Flask of Ichor - I forgot!). That was the hardest Mech Boss for me. I failed fighting it several times before the blade staff and once with.
UPDATE 2024-04-01: (no this is not a joke)
I just defeated Plantera twice, once with the Blade Staff, and once with the Sanguine. Everything else was the same. I couldn't see much difference. I dealt 700-900 DPS with both Blade and Sanguine when I was using the whip, Inferno potion, and 2 Flameburst Cane senturies (10-mana version). I remembered the Flask of Ichor this time. For some smaller enemies, the bats (Sanguine Staff) might have a slight edge? The blades shred Dark Casters, so I'd have to go with blades over-all. Sometimes having more than one kind of minion can be helpful?
