[Stretch]

Caelum's should do well with mammoths against any kind of armored indies. Against barbarians it can do pretty well with mammoths, or massed archers (enough to kill the barbarians before they get too close). The best advice to fully understand the capabilities of your national units is to run the first 10 turns of an expansion in single player (to determine how fast you can expand with a given set of strategies).

Also keep in mind that you can use a few sacrificial units to protect your important units from the initial attack of a group of enemies. You can put a high protection unit with a shield out in front to catch archer arrows, or put a group of units in front with orders "guard commander", and your commander in the back of your formation with orders "hold x5, stay behind troops" and the enemy force might chase it into the jaws of a trap while your flanking units crush them from both sides.

When testing out expansion strategies for Caelum, remember that you will want to start buying eagle kings every turn as soon as possible. Once you research some evocation magic (culminating in having Storm, Thunder Strike, Summon Storm Power available) you will see why!