Stop being crap at strategy and abuse pairing up to protect/strengthen your weaker units.
Robin gets a exp boost while paired up. If he is that accident prone, pair him up with Chrom or Frederick to prevent that from happening. Chrom has a skill that increases the chance for a Dual Strike too while you are at it.
Also, if Risen appear at the Longfort, always do it, it is the easiest map to grind on. Just send Kellam and a long-range unit (Any of Virion, Miriel, Ricken or Tharja) to the staircase to the south to block off the lower part of the map, and then go on a full-out brawl with the rest of your team on the upper part.
Also, on the map where you recruit Donnel, he only need to get 3 kills+1 attack in to get a level up and thus to join the party. Out of the 25 enemies on that map, it should be doable to plink a few down to low enough hp for Donnel to finish them. Get someone to pair up with Donnel when he needs help finishing opponents. You probably need to carry him when grinding though, but once he hits Level 15 and promotes, he will be one of your best units by then.
The enemies on the map consist of Barbarians, Archers, Mages and Thieves. Archers can't counter-attack up close, so they are the easiest finishers for Donnel. Thieves give a bit more exp, but they will double and off Donnel if he misses. Mages are squishy, but deal more damage if they get the drop on Donnel. Last of all, the Barbarians have a weapon advantage against Donnel. Long story short, make sure that no mouthbreathing bandit gets the drop on him. In general, having units with good speed really helps on this map. Get your sword-toting members on the right level, and they should be able to dodge most Barbarians' attacks. Keep in mind, terrain advantages are your friend! Also, the weapon triangle. Wielding a sword gives a nasty hit dip for the enemy Barbarian or Fighter.