XCOM: Enemy Unknown

The following six types of alien ships must be shot down during the same playthrough.

Small: Small Scout

Medium: Large Scout

Medium: Overseer

Large: Abductor

Large: Supply Barge

Very Large: Battleship

• Progress until near the end of the game under the Classic difficulty setting. Note: You will need to save and reload the game frequently. Use a sniper in a flying suit, preferably with Psionic abilities and the Double Tap ability. Equip him with a plasma rifle and wait for the small alien craft to appear. Shoot it down so some aliens will die and save some time. At the start of the mission, fly up with the jetpack and move towards the alien craft. The Double Tap ability will help you to kill two enemies in one turn, and the flying suit will help kill them from a distance. However an Etherals can mind control you. Frequently saving and reloading will solve this issue.
• Select the Classic difficulty setting. If available, use the "Second Wave" option "Save Scum" to allow new seeds if you get a bad outcome. Develop and build the Hyper Wave Relay and have Colonels. The Hyper Wave Relay lets you know exactly what will be encountered at a crash site, and thus avoiding getting surprised by Etherals. Do this against a Small Scout, which typically has between five to twelve aliens on board. Foundry should also have been built. Play as a Heavy with Ammo Conservation to minimize reloads and Deep Pockets to carry extra grenades. Also set up with Bullet Swarm (shoot twice), Shredder Rocket (extra rocket), Rapid Reaction (second shot on Overwatch after a hit), Grenadier (with Alien Grenades), Rocketeer (additional normal rocket). Equipment should include Heavy Plasma, Blaster Launcher, and Titan Armor. If playing with Enemy Within, also consider Neural Dampening, Hyper Reactive Pupils, and Forget the Meld.
• Play under the Classic difficulty setting with Ironman mode turned off. At the start, shoot down a small UFO with only Sectoids and a Commander. Save the game before sending in your Sky Ranger in case the map has a bad layout. If you die you will lose your auto-save. Select Squaddie Heavy to be the lone wolf. A medikit can be useful, but a grenade will be more effective. Save the game upon landing to scout, then reload if you need to change the layout. Try to find a map featuring two groups of two Sectoids and a Commander, with them spread far enough out so you can engage just one group at a time. When ready, approach the first group of Sectoids. When it is their turn, one will usually Mind-assist the other. If you are well placed, it will usually miss. Reload and try a different approach if needed. Then, shoot the assisting Sectoid as this will kill the other one as well, then repeat the process for the other pair of Sectoids. Use the grenade if necessary, but save your RPG for the Commander. The Commander has more health and will usually survive after the being shot. Keep far enough back so that after he moves, you can still see him, but he will miss you on his turn. You will require a full turn to fire the RPG. You can survive being shot once; use the medikit to help you continue, or use the grenade to finish the kill.

Choose a soldier at the Gene Mods lab and modify his legs with "Muscle Fiber Density". In the next mission, that soldier will be able to jump two stories high.

 The Bulldozer must be on fire for about ten seconds. This can be done in the Cook Off job on Overkill. Wait for him to enter the house.,then flame him from behind the kitchen counter. Have support with Molotov cocktails from upstairs to keep him on fire.

Successfully stun an Exalt-Agent with the Arc-Thrower. Do this by shooting the agent down to less than 3 points of health, then move close to him and use the Arc-Thrower.

The "special-duty" soldier is the one that is sent out alone on covert missions. When your team goes to extract him, ensure he gets three kills.

Use a MEC Trooper with the Kinetic Strike module.

Upgrade one of your MECs to level 3.

Note: This requires Enemy Within. The "Seeker" robotic flying squid has a strangling attack. Kill a Seeker strangling your squad member it kills them.

Kill an alien revealed by a nearby teammate with the Bioelectric Skin gene modification. The rest of your team should not be able to see the alien, and kill it with a rocket, grenade, or other weapon.