if you wanted to go for the beserkers idea, you could give them a something like an enraged promotion, equivelent of twice the dammage they've recieved. so at 75% health they get +50% offence, -50% defence, at 20% health they get +160% offence, -160% defence, but that doesn't mean much when they'll die in one hit.
at 100% health they have X strength
at 90% health they have 1.2*0.9=1.08X
at 80% health they have 1.4*0.8=1.12X
at 75% health they have 1.5*0.75=1.125X
at 70% health they have 1.6*0.7=1.12X
at 60% health they have 1.8*0.6=1.08X
at 50% health they have 2*0.5=1X
at 40% health they have 2.2*0.4=0.88X
at 30% health they have 2.4*0.3=0.72X
at 20% health they have 2.6*0.2=0.52X
at 10% health they have 2.8*0.1=0.28X
Hence you make the unit usable at lower health levels up to a certain limit, thus encouraging you to continue fighting with a wounded beserker unit, improving the offence initially, of course at a certain level the risk becomes much greater.