As far as the DLC goes, I donno, it sounds like lots of fun, I love the style of this game, the lore is pretty cool, and of course the gameplay is second to none. But I got my PS4 just over a year ago, I have about 30 games I bought. (most single player campaign) and I am planing to beat them all within a year. So I donno if I have time for DLCs.
Longarius was the hardest boss to me, but his an optional boss. The other one, just level up some more, but dont do like me, i leved up a bit too much and the final boss ended being too easy.
For the ones saying to summon someone, its really hard to find someone to summon for those 2 bosses. I tried to summon ppl to help for longarius and never found anyone when the game was fresh, so i imagine now just if u request help from a friend. Eventually beated Longarius after a few tries.
Leveling up is the dificulty meter in this game, the more you level up the easier it gets, so my advice to someone that plays solo is to level up like 5 level and try the boss again, if the boss is dealing way too much damage to the point it 1 hit kills you you really should level up some more, if not is just a matter of studying his paterns.