Don't go in and attack straight away. Go invisible and just wait for the right opportunity. Once the AI focus is on your tank or summons you are less likely to be the point of attention.
The rogue has skills like Eye Gouge (blind a target), Trip (Knock down a target), Razor's Edge (Stun a target), Charming Touch (Charm a target) which can seriously change the battle in your favour. Maybe look at applying these skills to your enemies first, to change the battle in your favour first, and then start killing things.
While invisible if you can position your self near 2 or 3 targets, use charm first to make them attack each other followed by a disable spell on another enemy or just use tactical retreat (expert marksman) to move to a lone enemy which will be bread and butter for a rogue.
I'm currently playing a rogue on hard difficulty, and using the following build is working pretty good for me:
Man-at-Arms x 5 with the following talents:
- Thick Skin (Armor)
- Picture of Health (Vitality)
- Weather the Storm (Resistance)
This will help your rogue survivability.
Backstab with Bully, but without Guerilla (I'm lazy - extra backstab just as good :D).
With the following non-rogue skills (in addition to the rogue spells i suggested for you):
Oath of Desecration - Witchcraft (int)
Rage - Man-at-Arms (str) (only use when they knocked down)
Melee Power Stance - Man-at-Arms (str)
Tactical Retreat - Expert Marksman (dex)
I'm doing damage outputs like the following in phantom forest at the moment:
https://www.youtube.com/watch?v=qpAMpSCEgQ0 If you come across a difficult battle, maybe consider getting out summons first and then using shields on your party members (earth, fire, water, air - one for each party member :D). This will allow your party to withstand more damage for a few turns in which you should really eliminate a few enemies in order to change the battle in your favour.