Sealing jewelry is a very good idea. The benefits aren't huge, but neither is the cost.
Ok, I've finally beaten Bittercore a couple times now. The first time was at 28. I think my method probably could have worked at 27, but I didn't get around to retrying until I had already dinged.
I've now dinged 29 and am working on hitting the cap.
And wow, the Mainframe was rather anti-climatic after defeating Harbinger.
Anyway, here's what I chose to do. And I ended up even using my level 20 potions, not the 25s. That was an accident. The 25s would have been better, and as OSUBeaver suggested, the purchased ones would be even better.
I died 3 times still on 27, but that was due to experimentation, rather than because I was outclassed.
I re-skilled and added EVA buffs. I also switched from the Tier 3 Chakra attacks to Tier 4. Tier 3 is a bit better for party-based PvE, because of the reduce aggro stuff, but you don't need it soloing, and Tier 4 is a lot better for PvP, and a bit better for soloing.
First, clockwise around to Thanatos. My DPS now is pretty high even outside my mech. I died a couple times here trying a couple different things.
In all cases, pop any buffs you have, including potion buffs. Kill the Lupi. You might have to sneak around to find him for Harbinger. I also have my pet out.
First, I went mechless and went claw-to-claw against him. I just tried to out DPS from beginning to end. When he did his big attack at the end, I still had a decent amount of HP, so stood there and took it. Bad idea. Dead.
Second, I tried the same thing, but ran away at the end. I didn't make it though. Could be I waited too long. Also, my health was lower this time. The random number generator wasn't as kind to me, I guess.
Third time, I tried Thanatos with the mech. I didn't even do a Fade/Shadow Strike/Mech, as I usually do when using the mech. The mech gives me two advantages. First, I run away faster. Second, if I start out with the mech, I can pop a heal as soon as I switch.
Took him down easily.
Now around to Harbinger. Same strategy as with Thanatos, except for the running away part. Go mech, and stay there. I wasn't watching my CP and I did run out at the very end, but survived. Would've been better if I could have stayed in mech the whole time by popping a CP potion.
After Harbinger I never mech'd again. Around to Azrael, and then the Mainframe. Hack and slash. Easy.
Oddly, the first time I killed the mainframe, the Predator Bot never appeared. Didn't happen until my next run through on BC. And it sure surprised me. Almost killed me too, as I was having a hard time targetting and my six year old was asking me questions.
One other thing I did for the Mainframe battle. I turned on the Battle Chat. You can't always see the little bots that come up from behind and start attacking you, when you're pounding on the Mainframe. But if you start seeing damage appear in your chat from somewhere else, you know it's time to go take care of the nuisance.