I also find it exceedingly hard to believe that it's dying in 2 staggers only.
I know that people have to use chainspell at the right time, I'm not an idiot.
I think you're exaggerating and setting an unrealistic standard of what to expect from the Botulus Rex fight for people who haven't fought it before.
It is entirely possible to kill B.rex with 2, or even 1 EV/HQ stagger only with BLM tank method, although your pt needs extremely high dmg output.
There are some tricks to increase the dmg output of the pt:
1. Have at least 5 BLM(some ppl do 6 even, but I usually do 5 to include more jobs for proc), while 4 BLMs in BLM pt focus on DD and another BLM focus on proc.
2. When you invite BLMs, leader have to make sure they have capped elemental skill and magian thunder staff. JP pt leads often make sure BLMs meet the requirement, that's why they never fail.
3. Before you pop the NM, COR x2 can do roll rotation and buff the BLM pt with mattk/macc(or fast cast) roll, then disband and go back to melee proc pt and buff TP roll x2. I honestly never play BLM, so I'm not entirely sure if macc is more beneficial or fast cast. I'm assuming macc if you're using magian staff maybe?
4. Have WHMs /SCH and boost AGI+Firestorm WF COR and RNGs. WF can hit 3k avg(avg, not spike) on Rex without firesday/fireshot bonus if properly buffed by WHM. On firesday it's entirely possible to hit 3.3k+ avg.
5. WF COR and RNGs need Atmacite of slayer+save TP atmacite lv 15, and karin Obi.
If all the BLM/COR/RNG/WHM meet above requirement, it's possible to reach less than 50% after 1st EV/HQ proc, thus only need 1~2 HQ/EV to win. The real problem is to find this many ppl that meets the requirement.