Your first question has a bit of complication involved in the answer for Att. Critical caps at 155 in Dullahan's raid (170 if you are a Lann due to Critical Break). I will come back to the Att part after.
Ein Lacher contributes 5 Balance and 3 Critical. 100 Neamhain kills contributes 1000 Att/M.Att, 5 Critical, 1000 HP, 300 Remove Att Limit. Bear in mind that the Ein Lacher stats come mostly from Gold medals, which require defeating the bosses while taking *zero* hits. It isn't easy.
Coming back to your question on Att...
Level 90 and 95 weapons have a possible stat that you can add to them, called Remove Att Limit. It is very VERY expensive, but it raises the Att cap by an amount equal to how much Remove Att Limit you have. It is only available on Lv.90 and Lv.95 weapons, and maxes out at 2000. Neamhain's kill achievements also grant an additional 300. This makes it a total of 2,300 with both.
But the problem gets bigger. The Att Formula is currently broken in all regions due to an oversight KR made. The Att cap is now independent of the stats of the enemies, and is now an arbitrary value set per battle. It is generally accurate, for older content, but not so much for recent content.
Dullahan has 18,500 Def, but despite this, his Att cap is 28,000. This means you need 28,000 Att to deal full damage to him, or 30,300 Att if you have the extra 2300 Remove Att Limit.