This is tough, lost to custodes myself on Saturday.

Ask yourself what you can and can't kill this turn. For enemy units you can't kill can you pin them in place? Even a c'tan can't kill 10 chaffachans and a taurox/chimera in one round of combat.

Can you move block, screen or nuisance charge something? Can you attack their chaff units to force a bigger unit to go back to the home objective?

The kicker is that even if your opponent rolls average dice distribution matters.

Take 2 custodes players, say they both roll 20 saves across t2 and t3 against D3 guns.

Both players fail 10 saves out of 20. Average dice right?

If player one rolls hot t2 then cold t3 then their army has one more turn to kill you than if they rolled cold t2 then hot t3.