Basically if you are dying a lot, try different tactics and experiment with a different mix of units. Positioning can help a lot, especially when the enemy turn comes and your units are set up in the right spot to kill them off with intercept fire. Try not to finish your unit's action with them in a vulnerable situation. Also, if you get frustrated, remember that you can save mid-battle so you don't have to redo the whole thing (hit the menu button on the map screen between moves).
Ignore rank; I feel like A-ranking missions is something you go back and do after the first playthrough, or save for skirmishes. Trying to rush through the maps the first time through kills a lot of the fun - for me, anyway.
