1. Test every available class in the game. There are no better and worse classes here, as all of them are useful in the right hands.
2. Always have at least one soldier of each class in your strike team. Ranger will be useful for reconnaissance, as well as for close combat, a sharpshooter will easily take out a target in the distance, and a grenadier will get rid of a number of enemies and/or their cover at the same time.
3. Remember about merits and flaws of each class - a sharpshooter will be the most effective while fighting from a long range, whereas a ranger will benefit from attacking enemies from a few tiles at most.
4. Some of the missions will have a time limit in which you must complete your objectives, but in general they are well-balanced - you can really take your time and you should still be able to complete them. You shouldn't waste 5 rounds in row by standing idly and overwatching, but you shouldn't either be rushing into your enemies without a plan.
5. When you are travelling in concealment, don't take the red markers representing your enemies' sigh range for granted (especially in cities and dense forests). Try to position your soldiers in such a way, that they will be covered by terrain from as many sides as possible (especially from that one you don't have revealed) - by not following this rule you will end up in a scenario in which an enemy group comes out of nowhere and ruins your best plan.
6. When fighting in cities, remember that civilians can break your concealment as well - try to avoid them if possible.
7. Concealment is broken when one of your soldiers performs almost any action (like throwing a grenade, attacking an enemy and even hacking), or if they simply make a lot of noise - by bursting through the door, or by smashing a window. Plan your actions accordingly so that you won't make a silly mistake.
8. Some abilities won't break concealment - you can use most non-offensive abilities, as well as scan the area, or even heal your units. Hacking in general is quite dangerous, as most of the actions you can perform while doing it (like taking control over a robotic enemy) will break your stealth.
9. Try to spend your resources (supplies) reasonably, as they are used for almost everything in the game - building facilities, purchasing new recruits and soldiers, constructing new equipment for your soldiers and so on. Try to always have enough supplies so that you can start the construction of a new facility.
10. Similarly to supplies, you should think twice when spending intel - don't just waste it on meaningless things and try to have enough (from 40 to about 80-90) to be able to start making contact with the resistance on a new continent.