I think Barcelona is not under much pressure in the Champions League platform now. We have seen Barcelona win four consecutive matches. Barcelona started the Champions League with a defeat. But due to winning four consecutive matches, Barcelona is currently in third place in the points table. So even if they lose one match, Barcelona will not face any major loss at the moment. However, Barcelona's main target is definitely to be in the top 8 of the table.
It is definitely difficult for Barcelona to win against Dortmund. Dortmund is performing quite well in this season of the Champions League. Dortmund is currently in fourth place in the points table. Dortmund has not faced any strong opponent other than Real Madrid in their 5 matches. And Dortmund lost against Madrid by a big margin of 5-2. Apart from that, Dortmund has played against teams like Celtic, Club Brugge and Sturm Graz. These teams are quite weak. So Dortmund got an easy win. It is also very difficult for Dortmund to win against teams like Barcelona and Madrid.
I am not at all worried about Barcelona's form in the Champions League. Except for the first match, they have won every match in the Champions League exclusively. On the other hand, it would be a mistake to think of Dortmund as an easy opponent. Like Barcelona, Dortmund is also in their best form in the Champions League. Both teams have played 5 matches, won 4 and lost 1, and are at 4th and 5th in the points table. However, Dortmund has performed poorly compared to Barcelona in the Champions League. Whereas Barcelona's opponents have played against strong teams like Bayern Munich, Monaco and Brest and have won matches exclusively against Bayern and Brest. On the other hand, Dortmund has won matches against easy opponents. They have not played against any other strong team except Real Madrid. They even lost the match against Real Madrid miserably by 5-2 goals.
Barcelona will definitely be ahead considering past performances. But we are definitely going to see a tough match. 3 points are equally important for both teams to stay at 8th place in the points table.