Any tips for tier 3 Expert Old One's Army?

[JKLamone]

TL;DR: My first time playing in 6 years is expert mode and I need help with Old One's Army.
So here's the thing:
I came back to this game a few months ago after taking a break for around 6 years. I started a character, played normal mode, then completely overprepared for the WoF fight, which led to my computer being a jerk and corrupting that world file. So I made a new character, new world, and decided to try expert mode. So here I am, my first time playing in 6 years, and I'm doing expert mode. Not too bad, bosses are a little harder, Duke Fishron is a pain (even though I beat him on my second try! YAY!). So when I got to hardmode I forgot about Old One's Army and accidentally fought Golem without building up enough defender medals, and now I'm stuck on tier 3. I've tried a couple of times with different methods, but every time I lose on wave 6 or 7. My current strat was going off of one of Yrimir's videos (

this one), and it got me pretty far. I've gone with full shroomite armor, ranger emblem, ninja gear, ankh shield, papyrus scarab (any suggestions to replace this?), magic quiver, and celestial shell. Currently, I only have the explosive trap cane and lightning aura staff. The potions I use are ironskin, regen, ammo reservation, and archery potion. I have my UFO mount out constantly so I can have that extra speed and flight. I also use holy arrows, like the video, but I still can't seem to defend further than wave 6 (although I did get Betsy down to half health one time).
Any other strategies or tips would be greatly appreciated, as well as anything I can be doing different, any different accessories, etc etc. Thanks for any tips you guys might have!

(also I'm still new to the forums so I'm not sure if this is under the right one, if not please just point me in the right direction, thanks!)

 

[Gotcha!]

Use weapons that fire in a straight line and pierce multiple enemies (like the Pulse Bow) and only use the Lightning and Ballista traps. The Explosive and Fire traps are rather underwhelming. I prefer Lightning, but others swear by Ballistas.
Change accessories that don't give you any direct benefits into accessories that do more damage. Try to get the Destroyer Emblem, Sniper Scope and Celestial Shell.
Don't use the UFO unless you're in danger or when fighting Betsy. Stand on the ground and use your armour's stealth mode to do more damage.
Make sure your arena is 100% flat, so you can stand in the center (next to the gem) and only need to shoot to the left or right. Make a small wall on each side, so that your Pulse Bow arrows (or other weapons that bounce) can bounce back and do more damage.

It's been a while for me, so that's all that comes to mind.

 

[JKLamone]

Use weapons that fire in a straight line and pierce multiple enemies (like the Pulse Bow) and only use the Lightning and Ballista traps.

Thanks for the advice. It actually really helped and I think if I was just better at the game I could've won using this strat. I decided to fight the celestial pillars first and get better gear, so I'm hoping now that I'll be able to beat it much more easily.