R.U.S.E.

Here is a list I compiled of each of the secondary objectives, and I have given a short description as to how to gain each one, there are 43* in total, you will need to gain any combination of 21 or so to gain this achievement:

Mission 1 - Prologue - (1)
(All 3 of your tanks survive the level)

Mission 2 - Taking Command (1)
(Reach Kasserine in 3 minutes) - Just rush towards it after the objective appears, your units should have little issue handling the enemies.

Mission 3 - Kasserine Pass (1)
(Make sure no anti tank guns are destroyed) - There are a total of 5, I reccomend that you place one at each exit of the town and alternate the remaining one to the exit you feel is in need of it. Move the infantry accordingly to where infantry are attacking you from.

Mission 4 - Wild Bunch (1)
(Fall Back without losing a tank) - You'll have to send your light tanks forward at a part in the level, as soon as you see the medium tanks though you'll get this objective, simply send them back to Kasserine when it pops up.

Mission 5 - Digging Out The Fox (2)
(Activate Spy Ruse) - Just activate the spy RUSE in the designated area.
(Destroy the Tiger tank) - The anti tank units should make light work of this and it should fall within seconds.

Mission 6 - From Bait To Prey (2)
(Find and neutralise hidden infantry) - These are infantry hidden in forests, use your recon to locate them and then defeat them with your own infantry, be sure to use the forests advantage in the battle.
(Use the Spy Ruse) - When asked to play this RUSE, play it on the sector that it asks you too.

Mission 7 - Hide & Seek (1)
(Preserve your M3 Lee tanks) - Be conservative and try not to use these tanks if possible, or at least use them to a minimal extent.

Mission 8 - Tangling With Barbara (1)
(Capture the 3 enemy field bases) - Use infantry to capture the designated bases.

Mission 9 - Battle Of Montecassino (4)
(Activate the blitz ruse) - Activate specified RUSE when prompted.
(Clear the way for the French) - Clear an opening for the French troops to get through so they can progress towards the enemy HQ. Just bomb the enemies blocking them.
(Clear the way for the British) - Same as for the French, just persevere to clear out all the opposing enemies.
(Don’t let your allies lose more than 50 units) - Just bomb the HQ as quickly as possible when this objective pops up to prevent your allies losing many troops.

Mission 10 - Utah Beach (2)
(Relieve the 101st Airborne division) - Move an infantry unit into the town.
(Relieve the 82 Airborne division) - Same as before, move an infantry unit into the town.

Mission 11 - Hedgerow War (2)
(Destroy all AA guns),
(Destroy the heavy bunker).

Mission 12 - Battle Cherbourg (2)
(Use the fake offensive RUSE),
(Destroy all AA guns).

Mission 13 - Screaming Eagles (1)
(Recon city squares before dropping paratroopers, 7 in total)

Mission 14 - Death From Above (2)
(Capture the barracks) - Far left, use the heavy artillary to destroy the AA guns in the forests, then send paratroopers,
(Capture the German Secondary HQ) - Far right, use anti tank units and heavy artillary to destroy their first wave of tanks, then drop some paratroopers into the city near the HQ, these should ambush the tanks closest to the base as they come through.

Mission 15 - A Bridge To Far (3)
(Capture the armor base) - This is in the top left, send a paratrooper as soon as the mission starts and capture it, when the objective pops up you'll get it,
(Revert your frequencies) - Simply use the RUSE,
(Preserve all allied tanks) - Really easy, you need to get 15 through without being destroyed. Use blitz to get them through faster and just use a combination on soldiers in woods and fight-bombers to defend.

Mission 16 - Wacht Am Rhein (1)
(Destroy Skorzeny's commando) - After they appear, they rush the south of your base, upgrade your GI's to rangers as soon as possible and then leave these in the forests slightly south of the bridge, his commando will run into them.

Mission 17 - Siege At Bastogne (1)
(Play Fanaticism RUSE) - Simply play this RUSE anywhere when prompted to do so.

Mission 18 - Blood & Guts (3)
(Use the camouflage ruse) - Use this RUSE after being prompted to do so.
(Destroy enemy bombers) - You just have to destroy the 3 enemy bombers that come at you, send fighters or fighter/bombers after them to destroy them.
(Protect the priests) - Your ally will have 8 priests that you need to protect, just use fighter/bombers to destroy the opposition.

Mission 19 - West Meets East (4)
(Play the Terror Ruse),
(Destroy the 3 Wirbelwind tanks) - left side going up to the capture point, the recon plane should be able to find them to revel the secondary objective),
(Destroy a bomber plane near the capture point) - This is pretty easy, as long as you've kept your anti air units in tact you should have no issues with them destroying the bomber.
(Destroy the anti-air artillery on the right side) - Now for me to get the antiair objective I needed to go straight up from the spawn point to the little island and then hang a right over the bridge and then it showed up. I never got it to trigger going from the top (near the capture point) to the right or from the bottom right up the right side).

Mission 20 - Riding With The Devil (1)
(Destroy all the Maus) - These are some pretty dangerous tanks, however, if you take it slowly progresing through the level you should have little problem building enough units to counter these, and the level doesn't finish until you capture the objective so you aren't timed to do this

Mission 21 - Secret Weapons (3)
(Clear The way for the flammpanzers) - These will be to the far right, simply use two fight-bombers and a recon plane and blow up the units marked,
(Play 'Decoy Building' plan) - Use any of the decoy buildings RUSE's in the designated sector,
(Clear the way for the panzerjagers) - These are to the bottom left, use the same method as for the flammpanzers.

Mission 22 - Into The Bears Jaws (3)
(Capture the Infantry base) Top left, drop some paras and grab it,
(Capture the armor base) - Bottom left/middle, bomb all but the armor building, drop some paras and then grab it,
(Capture the abandoned HQ) - Far right, just drop some paras next to it or in the forest, then run in.

Mission 23 - Grand Finale (1)
(Preserve the second front line (set up some infantry in the woods, and then use a couple of bunkers, you should only get attacked by one wave so you should handle them, use this side for sending the nuclear long tom too)