In Centurion, the trick is to defeat the enemies with minimal losses to your side and to their 
theirs. That way, you will have a readily available supply of fresh recruits as soon as your 
legion conquers that territory. If you play at Senator or Emperor level, you will notice that 
the number of men in your cohort is drastically reduced even though the size of your legion 
hasn't changed. As a result, your cohort will always loose on a one-on-one battle against 
enemy units. So your standard balanced-army-stand -and-fight tactic no longer guarantees 
victory - even against a smaller enemy army. Try going against Carthage with that tactic 
and watch your entire consular army get decimated within a matter of seconds.

Just like in real life battles, the general rule of the thumb to defeating the enemy with minimal 
casualties is to surprise them. In Centurion, this means flanking each individual unit with your 
cohorts. If you do it right, the enemy unit will panick and run away before they can even put 
up a fight regardless of how small your cohort is. Do this for everyone of their unit and you 
can win most battles with 0 - 100 casualties even at Emperor level.

This is a lot of work and requires you to manually set up your army and understand a bit more 
about the game algorithm. Basically, the computer will perform it's preset manouver (generally 
frontal assualt) and will continue to advance in the straight line until a certain point when 
they Melee and attack the nearest units. So here's what you do, and this formula works well in 
almost all scenarios is using a balanced army: wait until the sequence starts and then pause, 
move 3-4 cohorts on each of your flanks to the point where they can not be reach by the left and 
right most enemy unit (we're assuming that they will continue to march in a straight line) and 
make them form an ambush column. If the enemy General is charging infront of his troops down the 
middle of the field, keep two cohorts in the middle to give the arrogant moron what he deserves 
(and with a bit of luck, half of the enemy forces will panic when they see your troops stomp on 
their revered leader to death). Otherwise, tell the remainder of your forces in the middle to 
fall back. The enemy troops will of course continue to charge straight down the battle field and 
pursue your center forces. If you do this right, what you should see is your legionaires lined 
up in U-shape formation and the enemy charging right into the middle of it. Next, as the left 
or right most enemy unit is close to the ambush column, tell the cohort at the tip to advance 
into the approaching enemy unit's flank. This is a very delicate process and requires excellent 
timing. You basically want your cohort to attack the enemy unit's flank just as it enters the 
kill zone. If you do this right, the enemy unit will continue to march down in a straight line 
and in oblivion as your cohort hacks at it. They should panic in no time and start to flee. If 
you attack too soon, however, you will unwittingly march your men into their war path. If that 
happens then you might as well kiss that cohort good bye. If you attack too late, the enemy unit 
will turn around to face you - and since you can not win a face-to-face combat against an enemy 
unit at Senator or Emperor level, you can kiss that cohort good bye too.

Assuming that your first ambush goes well, the enemy unit will begin to panick. Waste no time 
with them. As soon as they panick move your second ambush unit into position to strike the next 
enemy unit in the exact same fashion. Remember, 2 cohorts can theoretically take out 5-6 enemy 
units if you keep attacking their flanks and keep them working as a team. Work your way from 
both sides towards the middle of the enemy ranks. Keep pulling back your central units to 
distract the troops into thinking that you're retreating and the field is theirs. Never attack 
any units head on because they will just anhilate your central forces. Before you know it, your 
ambush columns will converge on the last enemy unit and the battle field is yours - for little 
or no casualties. 

Sounds familiar? This is a more agressive version of the Cannae tactic - and if I get my history 
right Hannibal used this strategy quite often in the second Punic war to destroy eight Roman 
Consular armies (nearly 300,000 legionaires - a consular army actually contains 3-4 legions as 
opposed to the game version). Do not melee your troops even if you're sure to win - you'll be 
surprise at how much damage a lone surviving enemy unit can inflict when your soldiers break 
formation and run amock.

If you have them, your cavalries cohorts can be your best friend. Release them early on in the 
game and get them to flank the enemy. If you time it right you'll be amaze at how quickly they 
can route one enemy unit after another with no casualties. If the enemy have cavalries, deal 
with them first. If the enemy general is near by and not charging down the center, take him out. 

Elephants are nasty. At Senator or Emperor level they will kill every single men in your 
infantry or cavalry cohort if you pitch them face to face against each other. However, they are 
even more vulnerable in the flank than their infantry counterpart, and if you play your cards 
right you can get them to panic, turn around and charge back into the crowd of advancing enemies 
troops - inflicting even more damage. Here's the best way to deal with them - if you can, 
try to quickly line your cohort up into several columns and let these elephants pass through 
the gaps. Then attack their flanks and they will immediately panic. Don't kill them. Just let 
them retreat into the enemy hordes and watch them kill their own troops. Then reallign your 
troops and deal with the surviving infantry. Again, the was how Scipio Africanus reversed the 
tide of the 2nd Punic war at the battle of Zama (Scipio was actually more creative with this 
strategy because he actually told his legionaires in the column to sound the trumpets when the 
elephant get near so that they will panic and flee - which they did).

If you deal with Parthia, where you're facing 15-16 cavalry units, then be prepare to lose up to 
three legions. As it is with history, Roman legionaires always fare very poorly against 
cavalries - and there's nothing you can do about it but take the losses. Use your own cavalries 
to limit the damage and wear down their cavalries as much as you can by flanking them. But at 
the end of the day it's going to be a costly victory.