is fire sunder a must for this build ? the sunder and +fire dmg dont work for tainted and only for blood boil fire damage?
nope, fire sunder is pretty small impact until you get some -% fire gear and maxed blood boil synergies so when you're level 97 and have defender's fire in mang songs + facets in helm, its a pretty big impact
leveling up with mediocre gear you can do without sunder entirely, just using phys from blood boil + bind dps + bind amp to kill immunes and its quite fast
A half finished char might hit ~3200 phys on blood boil and deal 4800-6400 per cast against immunes after amp and have a bound demon that hits for 10-15k on its own (before amp). A finished char might hit as high as ~6000 fire / ~4800 phys with 40% pierce and deal 9900-12300 per cast against immunes after amp and have a bound demon dealing 20-25k before amp. So maybe the damage on blood boil gets doubled overall against immunes when you bring in sunder and fire pierce
when you don't have fire pierce and have a lower level synergy/boil you're getting a 5% fire damage from boil and 5% from tainted and doing nothing. Although lesser monsters might still die to 5% damage on tainted, you can see it happen with weaker immunes