where can I find this mob? >> Dimension Invader Soldier lvl 40
Dimensional rift. For more info put it on google. Just to give overview: you need to be signed to 7s, you need dimensional fragments to enter the rift, its a hard spot to solo (there are chambers full with mobs HP x6). There are few different rifts: 20 - 30lvl, 30 - 40 and so on.