To predict any outcome you need know five things.
1) Field Length, this is easy to find, if you're using cats it'll be 1100+the boost to your cats range from tech, just attack a level 1 oasis to check.
2) Unit Speed, this is crucial when combined with field length.
Cavalry = 300
Infantry = 150
Archers = 125
Catapults = 50
this will always be the same with oases and forts however this may change with players so be careful with players as both tech and attributes affect it.
3) Range, this is easy and only two units are of note with range, cats and archers. Cats have a base range of 800 + 16 for every shooting skill level while archers have a base of 600 + 12 for every shooting skill level. This combines with speed and field length to calculate how many turns you can hit enemy units before they reach you. It it is possible, due to cav having 40 range and inf having 25 for cav to hit inf without retaliation but you have to work it out very carefully using speed, you only have 15 range sweetspot to play with.
4) How many units you can kill per turn, to work out this attack a fort/oases with plenty of the unit you are checking for (or use a small amount of yours instead) and check the war report to see how many eg with 200 cats I killed 407 inf so I can multiply that to find out how many if I change the number of cats, you will only need to check once each time you change your attack.
As a rule, unless you have checked, assume that cats will kill 1.9 inf per cat and 0.4 cav per cat. It will be roughly 1.9 cats/archers per cat due to lower range and health.
5) Max number of enemy units, this can be found out by scouting the enemy, with forts or players with lower scout skill it will tell you exactly (beware of hidden troops when attacking players, by this I mean troops hunting or at an oasis that can be called back right before you hit to stop you withdrawing until it'll cost you 5 coins). With oases and players of higher scout skills you will be given an accurate but vague answer.
"Several" means that enemies number is between 0~9
"One or two dozen" means that enemies number is between 10~24
"Dozens" means that enemies number is between 25~49
"Scores" means that enemies number is between 50~99
"One or two hundred" means that enemies number is between 100~249
"Several hundred" means that enemies number is between 250~499
"Hundreds" means that enemies number is between 500~999
"One or two thousand" means that enemies number is between 1000~2499
"Several thousand" means that enemies number is between 2500~4999
"Thousands" means that enemies number is between 5000~9999
"Over 100 hundred" means that enemies number is between >=10000.
Now you have all the info you need, using 1,2,3 calculate how many turns you can attack each unit type (use 2 to work out when they will come into range, if you are waiting the gap they must cover is 300 for cats, use 1 to work out how long until the units reach you and 3 to work out how long until their cats and archers can attack you) , calculate how many you will kill in that time by doing 4(the fourth point not 4)x[the number of turns you can attack] bearing in mind you can't attack, for example, inf all 7 turns because they aren't in range until 2 and you will be focusing on the fastest unit, cavs until all cavs are dead. Using 5, (take max possible for oases or vague player scouts) work out if you can beat all enemies before any reach you, remember you have inf as a last line of defence to, using higher speed, attack any enemy inf first and defend your cats/archers. Now you've worked out if you can win, if not and to show you an example fight assume I am attacking a level 8 oasis.
250 cats (Level4 Shooting Skill)
Max 2499 inf
Max 99 cav
Max 49 archers
Max 99 cats
Turn1: Wait with all units, set cats to hit cav. Result: all enemy cav dead.
Turn2: Leave waiting (do this all fight until I say otherwise) and set to hit inf. This is because, although archers and cats are higher priority there is a gap of 300 between your cats max attack range and enemy starting point so only inf will be in range. Result: approx 500 inf dead, 1999 left (assuming max).
Turn3: Set cats to attack archers. Result: all archers dead. Archers will be able to attack you before any inf or cats due to their 125 speed and 600 range so these are your top priority after cav.
Turn4: Set cats to attack inf. Result: approx 500 inf dead, 1499 left.
Turn5: Set cats to attack inf. Result: approx 500 inf dead, 999 left.
Turn6: Set cats to attack inf. Result: approx 500 inf dead, 499 left
Turn7: Set cats to attack inf. Result: approx 500 inf dead, all inf dead.
Turn8: Now you are probably wondering why I haven't done anything to the enemy cats, simple, the field length is 1164 (remember shooting tech) so the oasis cats (50 speed) won't have my units in range until turn 8 so I can ignore until then. That means on turn 8 I'll use my superior speed to attack the enemy cats. I will hit first. Results: all cats dead. I win 0 losses.
*Note, this was done before I hit a level 8 oasis and my subsequent attack backed up this example.
Turn order. No, I don't mean 1,2,3 etc, I mean how a turn is carried out. A turn is actually carried out by all units moving in order then units attack in order. This means that, for example, a cat will move 300 by turn 6 so although it can't hit you thanks to tech you will hit it turn 6 if you wait as it will move the final 50 needed and then your cats get to attack. However, I would still use turn 6 to attack inf and leave cats until a turn they can hit me, this is because with 4 shooting cats won't reach me until turn 8, the same turn as inf so it's simpler to continue attacking inf until they're gone.
Shooting Tech. There are only four key shooting levels when dealing with oases/forts. These are 0,1,4 and 8. These are the reasons:
0 - At this range cats can hit you on turn 6 and archers at turn 4.
1 - At this range cats become unable to hit you until turn 7 and archers at turn 5.
4 - At this range cats are unable to hit you until turn 8, giving you an extra turn to clear up inf.
8 - This is the highest you will need shooting for oases/forts as this is the last time field length becomes a critical factor. Cats are unable to hit you until turn 9, inf until turn 8, cav until turn 5.
Last but not least: fighting players. The rules change from when fighting forts and oases. Players attributes, tech and hidden forces mean you need to rethink all five points. For field length, if they have a higher shooting skill, the field length will increase, to find this out you need to attack the player, either in the main attack or a sacrificial inf to find out exact troop amount if their scout skill is higher than yours, if they're not commanding then you will also see their speed, something to check carefully in the war report. One thing to be wary of is hidden troops. If you managed to get a detail scout report always check prestige, make sure troop numbers make sense for someone of that prestige. Also look at attributes, if they have a high defence you will kill less per turn and if they have high attack then if they reach you they could cause high casualties.
If you have any more questions ask me when I'm online or send me a letter when I'm not. I'm on S5 so unless you are you have to contact me by these forums. Hopefully with this guide you won't need good luck