I believe that unless you got good defensive gears, Doppelsoeldner is one of the worst Boss Killers, because he will get his @$$ handed to him easily on a low CON/low def build.
All that you state almost always includes other Classes to support the character in question. No what can Doppelsoeldner actually do without any Heals/Frost Pillars/Quickens/Passes? It still has high damage skills, especially Zornhau, but aside of that?
If you’re not full STR you get less boost from GungHo or without DEX, you’ll have a lot less boost on Cencentrate, so most people will opt to go full DEX/STR or a mix of both. Then they get hit by the lightning skill of Helgasercle(?, dunno if that one is the right boss) and die because they take 70-120k damage to the face.
Instead of killing the boss, the boss is the killer and the Doppelsoeldner is dead meat on the floor.
Murmillio is by far the better boss killer because he can actually survive the battle due to the extra shield defence, ability to block melee attacks and avoid some magic attacks. Sustainability is what counts in real bossfights, not only top DPS that’s on life support 24/7 [and thus can’t live without all those Chronos & Clerics].
If you’re talking about the paper-mache bosses in dungeons or ET and include support, at Rank 9 any Oracle C3 will be the top boss killer of bosses up to 18m HP with the support of Kabbalist C3 and enough “luck” to get a decent number via Gematria/Notarikon.
As level 6 Twist of Fate will remove 20-24% of the bosses HP and is capped at 1,99m HP, if this hit gets multiplied by Double Chance lvl 11 to at least 5 hits (if the target has less than 10m HP) or 5+ hits (if the target has more than 10m HP), the boss is dead in 3-5 seconds without the need for any investment in attributes or equipment.