Havoc 40 Veteran (Agile Fighter)
 Veteran Build - Warhammer 40k: Darktide 

Build I used to officially reach Havoc 40 in pub games. 

Use bolt gun as boss killer, conserve the ammo for only bosses and snipers. 

Always mark targets to regain stamina so you can have infinite stamina and dodge while running eliminating enemies. 

Use Infiltrate to massively close the distance on ranged enemies and perform a very large 100% suppression effect, then become essentially invisible for 10 additional seconds from -90% threat. Also use it to escape sticky situations and clutch revive 3 people, do regular revives, and lastly do objectives unimpeded. Be careful to avoid damage from enemies targeting downed allies when reviving while invisible.

Use smoke grenades to close sightlines of ranged enemies, especially if under immense enemy melee pressure with enemy ranged behind in a choke pointy sight line area. Also use smoke grenades on downed allies to prevent ranged enemies from killing them. Use smokes on feet or team to essentially make a bubble shield.

You can be very aggressive with this build, and it is intended to be so, you will have infinite dodge, stamina, and very quick toughness gain on specialist and elite eliminations, always mark specials before eliminations. Note that the stamina and toughness benefit from focus target benefits the entire team in coherency. Lastly, when a boss is coming, save up 5 stacks before focus targeting the boss. Always spam full auto bolt gun on bosses weak spot, if under pressure, use infiltrate to reduce threat rating by 90% in order to bolt gun the boss unimpeded. This bolt gun should delete bosses under sustained fire.

Spam weak spots kills with rapiers to regain dodge charges and stamina. toughness leading to infinite sustain.

+Stamina is in order to get a meaningful amount of stamina back from the +5% stamina gain per stack on marked elimination. 

+1 Wound is essential to not be 1-shot and also to be at 66% HP after a down rather 50%. There is a reason why the "True Survivor" accolade is so hard to get, 95% percent of games involve 1 or more downs. Also if someone dies with 2 wounds, they get revived with 10-25% hp while 3 wounders get revived with 50-55% hp. Very impactful.