Infantry:
Desert Infantry - at under 400 gold, I was shocked to find that they have good morale, stamina, and overall decent stats. When I compare them to the 400 gold infantry of the other civs, I found the desert infantry to be more powerful than most of them. It has slightly lower defense than the auxilia, but makes up for this with better attack, charge, and morale. In fact, it is one of the few infantry under 400 gold that have good morale yet does not charge without orders.

Numidian Legionaries:
These infantry are far inferior to their Roman counterparts, but they can hold their own if necessary. At the very least, using these will be the equivalent of bringing elephants to a Roman army.

Long Shield Cavalry:
An excellent light cavalry, with good morale and stats for its cost. On its own it can't beat heavy infantry and cavalry, but in a flanking move it can be devastating, especially when combined with...

Mercenary elephants:
Essentially War Elephants, but slightly less expensive and slightly more powerful (only very slightly), at least according to the Unit Comparison Chart at RTWH downloads. The Numidians may lack heavy cavalry, but with the elephants, they don't need any. Flaming arrows may cause them to run amok, but combine them with the long shield cavalry, and you can kick most archers to hell and back. They will run through two units of armoured hoplites before routing, allowing your long shields to mop up the distrupted hoplites. They can shoot arrows at hoplites. They own heavy cavalry which may rout your long shields.

Numidian camel riders:
At about the same cost (10 gold more expensive) as the long shield, this unit is almost identical to the long shield, but lacks the LS' secondary attack, and is slower, making it slightly less effective as the support of elephants. However, they do have the important power of owning cavalry. This takes away one of the weaknesses of the long shield, the heavy cavalry. Also, they are slightly better than the long shield in sand, meaning that for maps with sand they are even better than usual. A compromise can be a mix of long shields, camels, and elephants.

Numidian Cavalry:
An average cavalry skirmisher, nothing really glamourous in combat. However, they do have higher missile attack than any other cav skirmisher, and have good stamina and morale.

My general tactic with Numidians with 15k per person on large unit size:
4x onagers (2880)
3x archers (570)
6x Desert infantry (2340)
3x Mercenary war elephants (7470)
3x Long shield/Numidian camels (1530/1560)
And still have 180 gold leftover for upgrades.

I'll put the onagers behind the desert infantry and the archers in front. Everything not on foot goes to one flank. While the desert infantry can't form phalanx and thus don't have the awesome phalanx capability in frontal combat, they do offer more flexibility that way. I'll attempt to force the enemy to action with the onagers. With 3 elephants and 3 decent light cav on the flank I'll most likely be able to defeat his flank and surround the enemy.

I don't have total success with this tactic, but it does do fairly well - even the cav + eles alone can do significant damage. Once engaged, the desert infantry and archers won't last long, but if the flank can be won, that doesn't matter very much.

And the Numidians have a high degree of monetary flexibility. On very high budgets, they can go heavy on elephants; on very low budgets their cheap but powerful infantry and cavalry can dominate the field.

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