I was under the perception that only Baal and Nilathak dropped charms with high enough level to grab those vaunted skiller mods. I'm fairly certain i just got a warcry charm from a minion while on my way to Baal.
if your looking for a skiller +high life mods (etc pally/45lifer grandcharms) then reroll only hell diablo/baal/nilly
if your looking for plain skillers ( etc warcry/pally/light/cold grandcharms) then you can reroll anything from act3 flayer jungle to act 5 end ..
i personally only roll diablo/baal/nilly ...i find so many other grandcharms that have a chance to spawn skillers that i save my pgems for a lifer charm
Ilevel 50+ = plain skillers (plain mosters NM Great Marsh onwards)
ilevel 61+ = skillers with 31-35 life (plain monsters most of Act 5 NM onwards)
ilevel 77+ = skiller with 36-40 life (plain monsters Hell Lost City onwards; Pindle)
ilevel 91+ = skiller with 41-45 life (Baal ilevel 99, Nithy ilevel 95 and Diablo ilevel 94)
Boss monsters have a higher level than the general area levels, but that's a pretty decent guide.