If all you have to work with is crappy players. Than you do that, and allow everyone in your group to learn the content with each attempt.

Pre made groups can be just as bad as DF groups. The reasons FC's, and premade groups are able to complete the content at all is due to persistence. When you have a static group (Even with healers that stand in AOE's) as you fail you will all collectively learn the fights mechanics. Making it easier for that group in future attempts. The reason a fight like Titan is so hard for DF groups is because most DF groups have players drop out after a wipe. So you will always be playing with ppl who have not practiced the fight, and you will always be playing with potentially undergeard or under skilled players.

The only way to prevent the above is to not only build a static group of players that are available, but you must gear up that group equally, and that group must all learn the fight thru however many attempts that requires.