U can kill boss with spec u want legendary then before using coin u change ur loot spec. Its not that hard tbh unless ur guild uses personal loot (SICK!)Coins
Elisande drops Fire and Arcane relic and 3 other items. Which means you have a 20% chance to get the arcane relic. If you switch your spec to arcane/frost, that goes to 25%. DISCLAIMER: You risk getting a wrong spec legendary tho!
Same applies to botanist - switch off Fire so you dont get the shitty Fire blast relic, or off-Fire for Krosus to avoid Fire crit dmg if you dont need it.
Never heard of or saw legendary item drop from bonus roll