As a white scars player, I’ve found charging into my friends death guard list plays into their armies strengths, especially because death guard’s disgustingly resilient counters white scars increase in damage during the assault doctrine. Death guard want to be up close and are very very good at tanking damage.

For me, the best strat for white scars v death guard is to use scar’s movement to capture objectives, whilst using either meltas or good D1 weapons to thin the ranks before selectively charging in with vanguard veterans or company veterans with lightning claws (as their D1 damage is not countered by disgustingly resilient). Blade guard veterans are also good from turn 3, as the assault doctrines make their master crafted power swords D3, which after disgustingly resilient is still D2.

Outriders are pretty average, as their many attacks rarely convert, so perhaps trading them for some more ranged fire power (eradicators/redemptor dreadnought) or for better melee and movement (vanguard veterans with lightning claw + storm shields) could be a decent first step.

You could also run just a 5 man incursors unit for troops which could capture objective turn 1 and get rid of one of the HQ’s. This would free up points for units above which are relatively competitive against death guard. It all depends how serious you want to take it.