As the title says, This maid need novels with the MC building up an OP army or a group. (BTW please don't even bother suggesting those "Army Building" tagged novels,,, I've read em all)
Heavenly Jewel Change.... but he doesn't start getting an army until half way through. Maybe around chapter 125ish.
Undefeated God of War ... One Man Army + op elite friends + army Yo seriously what do expect to get recommended, only thing left are rrl novels I mean ppl spam tags like crazy, so the chance that all army building novels are taged and baged is high (you see the warehouse joke ? )
http://www.novelupdates.com/series-finder/?sf=1&tgi=306,6093&mtgi=or&tge=9522&sort=abc&order=asc + History's Number 1 Founder
You could try Life Reset on RRL. It's more kingdom/base building and doesn't have a ton of chapters but it should do. Also maybe try Atros Imperium, also on RRL. I don't remember tons about it but it should have some elements of that, also I'm not caught up so I don't know how it turned out, but it was decent.
http://www.novelupdates.com/series/in-a-different-world-with-a-smartphone/ He build ton of fuckin mecha army for his group on chapter 300+ tho the translation is far from that