I did the timed achievement at the first go with 6 deaths or so. I tried blocking, gliding, stealth gliding, mounting up, which all didn't work, and I'm pretty sure I got shot through a solid rock cover once. A LoS bug I assume. Still I had like 20 seconds left in the end but it was bit annoying. They could've made some NPC give some warnings, but they prefer not to. Some people like to find out things like that with dying a few times, but to some it may feel like trolling. ('ha ha you thought you could mount up?')