Those Concentration DCs aren't flat, they're those figures plus the level of the spell (or in this case the spell level equivalent) you're trying to cast. Might want to re-read the details on the warlock's invocations.
Regarding different kinds of challenges from what you're using...
There's an (allegedly) encounter level 15 one I rather like. Get 4 harpy archers working together. All stay airborne and no more than 2 are singing at any given time (no use getting the party immune to all the harpies' songs right away). Depending on who latches onto which harpy and who manages to ignore the songs, they maneuver themselves to pull the party apart and minimize cooperation possibilities, while all harpies focus longbow fire on the most dangerous-seeming uncaptivated character until it's taken out of the fight. You can adjust the CR up or down easily enough by using different numbers of class levels, and presumably optimize it at the same CR by using different feats and/or different class levels entirely.
If you want to play the hidden menace game, and darkness and invisibility aren't doing it for you anymore, try 2 dread wraiths and an illusionist 11 (not necessarily an illusionist, just anyone who can cast them effectively). Encounter level is 14. The undead spring attack out of and back into a wall, the ground, or both. The spellcaster throws debuffs at the party and control the wraiths can ignore (wall of ice for example). After stirring up some lingering chaos he starts coming at the party with decoy illusory dread wraiths to waste their readied actions and create a good healthy sense of paranoia. If you're worried that the party might smear the illusionist quickly, taking out the wraiths' support, hide him, and I mean actually hide. Seeing through invisibility won't break a genuine Hide skill check, and if you start with a Small race, give it the Darkstalker feat, and make it something like a rogue 1/illusionist 5/unseen seer 5, it can be surprisingly difficult to figure out where the illusions are coming from.