If he keeps going to ground i suggest anything that ignores cover. Denying cover is kinda the thing to go for this edition.
IG have the order that forces him to reroll successful cover saves. Take some Rocket Launchers/Autocannons ect. Mass them and go for taking down a squad a turn

.
I am not 2 familiar with
DA or
GK ... but taking 2 dread knights and charging his infantry would probably work out the best. Give them flamers and the jump pack. For
DA i would just get bikes and drive them into his soft tau targets.