With the exception of the deadly sphere and desert tiger, all the minions with 'bad ai' were added before 1.3, these minions feel more dated if anything. This is why all the top tier minions were added in 1.3+ (and I'm counting the spider since it gained the ability to latch onto enemies in 1.3).
I've never had issue with Terrapirsma's AI it may need LoS to start attacking things but once it does it completely disregards tiles until everything is dead.
I haven't used the blade staff yet because, I can't be bothered to grind for super rare items. What I do know is that the spider minion is extremly good at dealing with caves dungeons, and other tight spaces so even if the blade staff can't do caaves you got a super easy to obtain minion that can replace it for that job.
This is a bit more of a minor incovienience for me, I think the reason sentries are like this is because unlike minions there's no debuff to desummon them, there should be a proper way to de summon your sentries.
Also while we're at it I just want to mention how imbalanced the sentries are, you got the queen spider which is strong, maybye even a little too strong, you got the frost hydra which is quite weak since it is too innacurate for bosses, and the lack of piercing makes it underwhelming for crowd control, and even if you get all its shots hitting it barely out dps's the far more accurate queen spider, then you got the moonlord sentires which are better than the queen spider but quite underwhelming compared to the other moonlord weapons.
Then you got the DD2 sentries, the ballista and lighting aura are fine sentries, but the flameburst is a bit weak and the explosive trap is complete trash, also I'm a bit bothered by the fact that the queen spider is better than the t2 dd2 sentries which require you to kill a mech to obtain.
I'd be in favor of buffing all the DD2 sentries so that the t2 sentries are a bit better than queen spider, and the t3 sentries could also use a buff, especially since a lot of players find t3 OOA event to be too hard. Also buff the frost hydra to around the point of the buffed t3 dd2 sentries.
about Explosive Trap sentry, first thing is, every OOA sentries need their own set bonus to be good, while I testing each sentry, at first I think the same thing with you but I got an interesting results.
first thing I realized is Lightning Aura and Explosive Trap are better than dealing with both ground and flying.
Ballista and Flameburst have better range but when trying to attack flying enemies in OOA, they're more like "distracted" by them leaving you and Crystal very vulnerable to ground enemies and you know they're not good at aimming at flying enemies they may wasted all their time try to hit them, putting Ballista on platform can help but also greatly reduce the effectiveness against ground enemies making piercing projectile meaningless.
Ballista is somewhat worse than other in practice for OOA even with setbonus. Flameburst take less penalty when placed on platforms due to it's AoE and don't have to contact multiple target to hit multiple target.
Flameburst sentry with setbonus is actually very good outside OOA, having one of the longest attack range among all sentry in the game with good projectile speed,
it's the true generalist, not Ballista which is slow and shorter range until you got Valhalla which allow Ballista to attack ~25% faster than Flameburst when panic but again, Flameburst can do better overall dps.
When it come to dps, Explosive Trap can make a lot better dps than Lightning Aura, okay listen, with Red Riding Hood, it attack rate is around 0.5 second per attack(faster than panicking Ballista) plus having base damage near Ballista(only a few point away from it), it's one of the most powerful sentry I ever seen.
It can even solo MM Biome Mimic with proper use in less than 10 seconds or even annihilate MM Hardmode Dungeon which no other sentry can do it like Explosive Trap, it's one of the best sentry to spam because it dps alone can outdps Coin Gun with Platinum Coin as ammo. the only downside is short range it really limited it power especially against bosses, but since you can annihilate most of the thing that touch the ground, you're pretty much ground-version of crawltipede. also it's very good as a kickstart for Lunar Cultist because it dps is crazy.
in Master Mode OOA, Explosive Trap(with setbonus) is the most effective one, while Lightning Aura can only massively damaged enemies, it took a lot of space and many flying enemies can go off unless you have to feed them Ichor, Ballista need to constantly panic to be "reliable" like other sentries, Flameburst is somewhat weak but very good outside, Explosive Trap can do 90% of all the damage for you though entire event, the only thing that survive are mostly bosses and Drakin, and some flying enemies summoned by Betsy but most of them are heavily damaged.
while people say Explosive Traps need a buff, tbh I think it's a fair trade for it power, if it's too powerful, reduce base damage a little should help but I think it already good.
(.. wait why am I replying old message.. oh sorry)