Key things to know:
Bretonnia:
Brets typically will let you go first. If they do so, they get the Blessing of the Lady, a 6+ ward to their knights and it becomes 5+ for S5 and better. Except for their characters, the knights are never better than a 2+, so S4, S4 armour piercing, or S5+ can do a lot of damage to them.
They have a banner that can add 'd6" to their a unit's first charge. So be careful, you might think you are far enough away, but still get charged! They also have a horn that can prevent you from flying for a turn. One of their lords can take a vow that gives them heroic killing blow on all their attacks. So if you see a lord going for a steg or a carno, beware!
Their trebuchets are S5, so make sure to kill them with chameleon skinks or whatever very quickly. And last, they get ranks for 3 models wide with cav, instead of 5, and can attack with all models on the sides... so getting charged by a lance can be very dangerous unless your units are buffed. Either redirect his lances to get flank charges or make sure to cast combat buffs on units that might get charged!
Dark Elves:
Dark Elves are definitely one of the strongest books in the game. Very few models/units that are poor choices, but some definitely stand out:
Witch Elves. Core unit, pretty cheap, 3 attacks each, with poison, ASF, rerolling 1s to wound. With a cauldron, they are even worse, rerolling all failed to wounds and getting a 5++ ward save. BUT, they are frenzied, so you can move them all around the board with skink units and make sure it never sees combat.
Warlocks. One of the most undercosted units in the game. Level 2 caster unit with soulblight and doombolt, two really good spells. S4, poison, ASF, and a 4+ ward to boot! Definitely a unit to kill quickly with skinks.
Pegasus hero (or dark steed heroes). With just mundane items, these characters can get 1+ armour saves, making them very durable for cheap. Some nasty magical items can make these guys really nasty.
But it seems like your opponent focuses more on xbows and spears. Just remember that the Dark Elves all have ASF at a high I so they'll get rerolls in combat with you. Arcane unforging can help destroy some of those nasty DE items, either on pegasus heroes or that caster. I'm not sure what 'invulnerable' thing you are talking about, because nothing exactly like that exists in the Dark Elf book or the main rulebook. DE casters can take a sacrificial dagger that lets them sacrifice models in a unit for a chance to get additional power dice to cast a spell. There is a cloak that gives a character a 3+ ward against all ranged attacks, shooting, magic, etc, but NO protection against close combat. There is also a ring that gives the unit magic resistance 3 and causes the opponent to have a miscast if you roll double 1s when casting spells at or near the ring holder. But neither of these are particularly invulnerable although they do afford pretty good protection.
Remember that DEs are BS4, so at long range they hit you on 4s, but they have a multiple shots (2) so that's 5s, and if you have skirmishers, that's 6s. And at S3, they shouldn't do a a lot of damage to your skirmishers. And as long as they don't take shadow, your blocks can fight toe-to-toe with the spearelves... but if he is running a lot of spearelves, then there might be a good chance of shadow, and that could be scary!
Hope that helps, please ask me to clarify if anything doesn't make sense.
