What is the best way to use drill ground?

The drill ground is for setting up pre defined march slots, using heroes and troops of your own choosing / preference.

The most important ones are probably the first x 2 slots, one for the kong and one for the airship.

You can assign multiple march slots, but, generally only have one kong and one active airship, the kong defaults to march slot one, the other slots (2 +) default to the airship, but can be changed.

The airship slots normally have higher killing power (against in game monsters and threats) than the kong, even if the kong slot is displaying higher power (+ for the same tier level of troops) which makes the main airship, march slot, generally, the main (+ most useful / powerful) march slot, of all of them.

March slot x 3 with no active kong or airship (when you have x 3 active marches out) can be useful for in game monsters (the easier ones to defeat) or for reinforcing, gathering and such like.

March slots higher than x 3 can be used with a slightly different balance of troop type (for situational use).

It is not often you will likely use more than x 2 or 3 march slots, at the same time, due, to high stamina demands, an exception, being, attacking monsters in the oracle reef event, where every, active, march slot, you have, is likely to get used, in order to gain more points and where stamina drain is not an issue.

For anyone who does oracle reef events, setting up multiple (usable) march slots (you would not normally use) is very useful, once you enter the event.

Conversely any troops not allocated to a march slot and not actively attacking monsters, will defend your estate if you get attacked (during the oracle reef event) which does not apply, only, if you stay in the safe zone, what ever your play style is like, you will benefit from a large amount of troops in the oracle reef event.

The drill grounds can also tell you, how many total troops you have, what type and what tier level they are.