I have used this build, lvl6 zerker with katana/m-79. On suicidal it's a bit of a challenge because it takes like 3 nades to kill a gorefast but it's fun....till the FPs come out to play....
I call that one the BOOMzerker. Easy to achieve, and every effective.
Spawn as Zerker. Buy Katana. Save up ~$1,000-$1,200, depending on Demo level and, thus, discounts. Switch to Demoman and buy M-79 and Pipes. Kill.
The Katana is for pretty much everything that comes your way that your nades/teammates can't handle. Which includes Gorefasts and the occasional Scrake. There are a few important things to know with this build.
1) The "Earthshaker". Toss a frag grenade and shoot it with an M-79 nade the second it lands for a HUGE, highly damaging explosion. An Earthshaker can instagib Gorefasts, Sirens, and Husks if your Demoman level is decent
2) Scrakes are your nemisis. Don't waste nades on them. Either leave them for other classes or, if you MUST, use the Katana primary fire to stunlock them.
3) At least on Hard, FPs can be killed in fairly short order by even a Level 3 Demo using this setup. Lure it onto a pipebomb and when it's raging, Earthshaker. FP will gib. And it'll feel great.
4) Remember: Direct nade hits pretty much suck vs groups. Try shooting the ceilings/walls to get the most out of a splash. If you shoot directly or even "splash" the floor, you usually won't hit all of a group, but if you shoot the ceiling directly overhead...boom!
5) Use the Katana whenever feasable, as no ammo spent on a kill = completely profitable kill. This isn't a cheap build. Each M-79 shot costs $10, each Earthshaker costs $50 total, and every pipebomb is $50+. Expect to spend upwards of $500 on a full reload! Where you absolutely COULD kill a pair of $3 clots with an M-79 round, you LOSE MONEY on it. Whereas if you Katana both of 'em, you GAIN money.