Currently on stage 396, Skellies and Grim Reapers are the way to go, but don't over invest into them too quick, you need to put those souls into the other monsters to get the achievement boosts (At tier 2 they give 10% each, it's pretty big)
I would drop Wrath for Calm, I have Calm at 1.3k and Defense at 291.4k and my skeletons only take 0.000056% damage, which means 10T skeletons can kill around 150 quintillion monsters without taking a scratch (Trust me, that's GAINS)
Edit: Btw, also important, first monster outside of S and GR you should get to 51 souls is Wraith, their passive gives you levels over time (great for offline progression), which at first is weak, but leave it alone over some 17 hours with max Necromancy and you should be getting a level every 30 seconds, faster if you leave it alone for a little while longer and specially faster if you focus your generation power on them (but I would leave it on GR, exponential growth and all that)