UPDATE:
because of the recent armor nerfs, hunter muntions has lost a lot of its power.
due to these nerfs, it is ALWAYS better to use a pure heat [Ignis] build (this one) vs a crit [Hunter Munitions] hybrid. this is because there is simply not enough armor for the true damage from [Hunter Munitions] slash to outperform raw stacking heat.
this is especially true in steel path, where because enemies have gained more flat hp, the scaling of stacking heat procs on enemies kills much faster than the hunter munitions slash builds.

new player notes:

• you do not need to invest 6 forma into this weapon, since it should just be a stepping stone to something better. however, it is kind of the only flamethrower and I think that's pretty cool so I have invested a little more into it...

mods:

• if no arcane, use serration over bane. with arcane, can use thermite rounds instead of bane if you don't like banes.
• if you are missing some mods, use the next closest alternative to you, or use more base dmg. could also use crit mods temporarily if you are missing lots.

basic details:

• crit bad for heat dmg, primed cryo bad for heat chance (full explanations later in guide)
• ignis wraith heat ticks benefit from multishot twice - wiki
  

  ‎the best i can explain it is, first multishot is factored into status chance (as in, how many procs u get per shot), and then it is factored in again because beam weapons combine all the projectiles into one projectile beam (multishot times base projectile dmg) and this is what all the status procs use for their damage calculations. in other words, we get normal amounts of status procs from multishot and each status proc uses a base dmg value of multishot times projectile dmg, so we double dip on it.

quick notes:
why using crit is worse ?

• because building for crit uses two mods to make half ur shots do extra dmg but the other half do a lot less since they dont benefit from those two mods, and for heat dmg specifically it turns out to be more efficient to just [Sacrifice] the crit mods to fit in additional heat and base dmg mods which work on all shots. the math says so. we do lose weapon dps but we make up for it with a lot more scaling heat dps.

why no hunter munitions:

• having to build for slash takes 3 mods out that we could have more efficiently used to instead increase heat dps by a lot more
• especially when slash is overall significantly weaker after armor cap rework

why not primed cryo?

• doesnt increase heat dmg and makes heat less likely to proc (since viral gets way more share of elemental dmg)

i made a heat dps calculator: https://docs.google.com/spreadsheets/d/197O0zfjdtC0HKrZM_HqPbbX4_21msqggJRXvfxljHww/edit?usp=sharing

enjoy!