It has felt like, for a while now, that Bile Titans are more of a nuisance than a credible threat. With the addition of the new spore titans I am reminded of ye olden days of unloading a full backpack off Recoilless Rife rounds into a BT's face and doing nothing, good times.
I do think the Titans should be able to be downed with one shot but I think it should be more refined and potentially risky. I propose that the Titans' head takes two RR shots to destroy OR one shot if you can get it in its open maw such as when it spews or does it's aggro roar.
I also think the Spear should still be one shot kill to give it back it's utility as a heavy hitter.
Edit: I meant to add that this should come with a reduced spawn rate to compensate for the difficulty jump (at least for everything lower than D10)