First off, fusing the Kartikeya and Zeed from here are very good ideas:
http://www.gamefaqs.com/boards/997805-shin-megami-tensei-devil-summoner-soul-hackers/66039196
Bump Kartikeya to AGI 40 using a Nigi Mitama as stated in the topic and push Zeed to lvl 59 using Kartikeya. One user says to use Murmur to push Zeed past lvl 59 and inherit his skills but it doesn't work. At the very least use Kartikeya to get Zeed to lvl 59, and he'll have 29 AGI by doing that.
Anyway, Kartikeya should be using Makarakarn at all times for the bosses, and I had Zeed max out Makakaja for the party first then use Sukukaja 2-3 times. After that either guard or use Tetrakarn with him. Doing that, most bosses are easy.
Nemissa should be your powerhouse damage dealer with Megido. I don't think she learns Megido if she's not on the electricity path though, so just try to use one of her top magic skills otherwise if she can't. If I was in trouble I'd use Mediarama with here. Again, not learned on non-elec path lol. In that case try to get Mediarama from another demon and put it on a high lvl one.
In this dungeon, I recruited a Mot and he's awesome. He's lvl 72 and knows Megidola. When fully Makakaja'd, he was able to dish out 1000+ damage which was close to Nemissa. I got him in Corridor 5, before Beelzebub. He was my main damage dealer besides Nemissa for Beelzebub. Really helped a lot since Beelz has a crapton of HP.
Before Beelzebub I also fused a Marici (lvl 71) which had Mediarama. I bumped her to 28 AGI (she couldn't go higher for some reason) which actually wasn't the best idea as sometimes she'd go before him and I wanted her to heal after Beelzebub attacked so I didn't have to use Nemissa to heal. However, this wasn't that big of an issue and most of the time Beelzebub went first...at least after my first battle with him, lol. First battle she went before him too often while after that it was rare for some reason.