I've got a lvl 93 asn at the moment and she is a pure light trapsin so I can say that they are very effective killing machines.
The only trouble that may arise with immunes is if there are no corpses around to be exploded but usually this isn't a problem because my merc/shadow (lvl 35 woot) can take most if not all light immunes I come up against.
Well light sentry has 3 synergies whereas death sentry only has one synergy. That being said, I generally cast light sentry first and when several corpses start appearing i set 2-3 ds out and that usually takes care of the rest. Oh, and if you want to speed up killing even more then throw in a few shock webs for good measure.
I'd suggest not maxing the last synergy, and putting some points into fade. Remember it gives you pdr - 1% each point. So even maxing it is a good idea, and gives you far more flexibility when you don't have to worry about resistances. If you add some pdr equip - you can reach 50% pdr on hell. Also - use cloak of shadows for ranged attacks, as these kind of attacks are probably the most dangerous threat to you. To stop all the missiles is really a wonderful help, solo or in a party. Personally I dont use mind blast, mostly because my shadow uses it so often
All these suggestions will make a gameplay a bit safer, but it's your choice whether you want max dam or more safety