Help with build

I have a challenge, that i must make fire damage boss killer.

I want to make build that:
-Has good one target DPS (DoT or not, doesn't matter)
-Has minimum 5k life
-Is as cheap as possibile
-Can be upgraded well via equipment change

Build that cannot facetank bosses will be fine. I thought about sth like Burning Arrow because of stackable burning.

It feels like you didn't really explore the forum that much before posting this thread.

Incinerate Chieftain
Tectonic Slam Chieftain
Ignite Discharge Juggernaut/Chieftain
Another Ignite Discharge Juggernaut
Ngamahu's Flame Chieftain

If you mean you want your own build, then grab Path of Building and try to make something yourself. All the builds I posted are cheap and are designed to be viable league starters, which means they don't require anything but can be scaled.