[Boron]

In earlygame you could e.g.cast Body etheralize + luck on your elephants .
That makes 87,5% of nonmagical weapons hits fail and still 50% magical weapons hits .
The AI has normally really few magical weapons .
You can add a few vinemen/ogres .
They have the purpose to avoid routing of your elephants .

In midgame you could replace them by behemoths .

The hypaspists are very nice MI .

Arco is imo more a lategame nation though . In mp it is really good because you have big magic flexibility with them .
With the rushing ai and their only average troops they are though not so good for sp than e.g. Abysia , Marignon or Caelum .

For learning the sp game i would recommend chosing either an underwater nation or pan cw/ermor .
If you intend to play mainly mp though don't bother with sp and just join a few games with a few nations or if you really like arco so much you can even join a few games all with arco but varying your strats a bit in each game .