Just some ideas off the top of may head:
I don't think a planet with a magnetic field strong enough to kill would be plausible naturally. Your talking about a huge amount of energy to generate such a field, likely the crust would be molten if that much energy was inside the planet.
The earth's magnetic field strength is about 0.5 Gauss and that of an MRI upwards to 30,000. Strong enough to yank a thousand pounds sideways at about 25ft or more.
Based on the fact that you stick humans in a 30,000 Gauss field with no will effects, most biological effects directly on life forms can be discounted.
The major effects would be very large and dense radiation belts that would trap ionic particles. These might be visible in they grew dense enough and might reduce insolation. At the poles, they could funnel dangerous amounts of radiation to the ground.
The in-fall would produce a lot of radio noise, possible jamming virtually all frequencies all the time. Aurora would be huge and constant, possibly blotting out the stars entirely.
But a lot of cosmic radiation would be blocked which is one of the primary producers of DNA mutation so variation would decrease and evolution would slow.
Cosmic rays also play a key part in the formation of ice crystals at high altitudes, so weather and climate would be affected but how I don't know.
Such a field might attract ferric meteorites towards the poles until they heat atmosphere and but heat of reentry sending them past their Currie point and shutting of the meteorite attraction to magnetic fields. When they hit and cooled however, a 30,000 Gauss field might be enough to make the tumble along like tumble weeds.
In geology, magnetic materials are rare in nature. To be magnetic, a material must be in some from of crystal matrix because the field develops between the paired electrons of atoms, not with them. The covalently bonded single atom of iron in hemoglobin, for example, is not magnetic. If it were, an MRI would be a very unpleasant experience. Only meteoric iron and lodestones are the only naturally occurring magnetic ores of iron.
The plasma channels of lighting would likely be constantly deflected sideways. This might lead to greater charge differentials developing between the clouds and the ground if the lighting can't jump the gap until it grows monstrously strong. In any case, lighting will strike far off to the side of the storm. Lighting might equalize charges within the clouds and ground for a long tie until a megaton lighting bolt finally jumps between. Ground lighting might shoot along the ground from hill top to hilltop.
Animal life would likely widely employ magnetic navigation using lode stones in specialized neurons like migratory birds do. On earth, the lodestones only provide a bearing if you travel a few hundred miles. On the super-field planet it would provide information constantly. In addition such a field could provide not only direction north south but also latitude by measuring the up-down deflection as the field curved. In principle, it could be used to measure altitude as well as bearing.
Animals might be far more wide ranging as a matter of course being much better able to navigate back home. Scent might play less of role in laying markers for navigation.
I think, but will have to check that electrical sensing, like that used by sharks, would be more powerful and longer range. Animals that generated strong electrical currents, such as electric eels might find themselves yanked around when they tried to deploy the current. On the other hand, they might use it as a means of sudden propulsion instead.
Spontaneous electrical currents in seawater or wet earth would be much stronger. These current arise when ionic charges such a Na+ move in the water across the fields. Trivial on earth but at 30,000 guass much less so. Plants might tap these currents for energy
In terms of technology, iron tools would be hard to use as they would pull constantly towards the poles. Copper, brass and bronze would be the primary metals. If you shot an iron or steel arrow, it take off towards the nearest pole very quickly.
On such a planet, iron projectiles might be super weapon as long as you could move the things (keeping hot past their currie point would work) then place them side of the target opposite the pole, then let it cool let go.
However, it would be easy to make a generator. Just a hoop of wire stuck up perpendicular to the field and spun by a water wheel would generate a significant current. Conversely, ambient impedance (electromagnetic friction) would be enormous, draining power from any long wires used to transmit power.
For a motor, like a generator, you'd just need the rotor and could use the ambient field as the magnets. The motors would have to aligned with the field at all times and you'd have to put them in a gimbal to use them in a vehicle.
With a 30,000 gauss field you could probably "sail" by using electromagnets to pull or push from the poles. By adjusting the relative strengths of the magnets north/south and the force vectors they exert on the vehicle, I'm pretty sure you could get any vector you wished in a 360 degree arc except maybe perfectly perpendicular to the field lines.
Such a craft might have a vertical windmill with generator rotors inside generating current to an electromagnet. Or not, as the generator would cause a lot of friction against the field, possibly more than the electromagnets could overcome (no-free-lucnh.) Such a magnet would need a ferrous core so they would probably have to use heat to turn it off completely.
Such a craft might make use of aluminum and other diamagnetic materials that are repelled by magnetic fields (that's how the aluminum separator in a recycling plant works, ferric metals go one way diamagnetic go the other.
There is a powerful ground wave in the earth that is used for some radio frequencies and it strongly affected by solar storms and thus the magnetic field. I do not know the details but it might be possible to harvest the ground wave for power just by sticking conductors in the earth. I think Tesla worked on that but abandoned it because the field strength was to low (but again, don't remember the details.)
If you do want to look build a world with a wildly powerful magnetic field, I'd suggest investigating magnetar. These are neutron stars that spin so fast their magnetic fields are extremely powerful, enough to stretch atoms out into thin lines.
With stellar systems looking fairly unstable these days, you could postulate a system that forms from two other systems, one with a magnetarand one with a terrestrial world, the systems collide capturing a magnetar in a planetary orbit. Then a terrestrial world could get swept up as moon of the magnetar. Even at great distances, the magnetar's field would affect the planet. If the planets pole of rotation was largely perpendicular to the magnetar's field at the planets orbit, the fields would merge making the planet appear to have a super powerful magnetic field.
Such a field would impart a lot of energy to the planet's core leading to more vulcanism. The planet would be nearly completely immune from stellar storms and cosmic rays because the magstar would trap them all. The planet would have to be orbit above or between the magstars radiation belts.
In that case, you might have a field strong enough to fly in. Certainly, in space it would make magnetic sails a serious form of propulsion. If nothing else, you could launch radiation shielded spacecraft from directly over one of the magnetic poles.
A sudden increase of the earth's magnetic field to 30,000 gauss would have civilization wreaking effects. First, all long distance power lines would fail, likely most generators and electrical motors because the earth's field would swamp the fields of the magnets in both. Radio noise would jam most frequencies. Most electronics would fail due to increased impedance.
Great herds of junk cars would roll off towards the nearest pole. Skyscrapers would face lateral forces they were not designed for. Knives would fly through the air and guns would, for the first time in history, kill without human agency as they shot out of cabinets and bludgeon people to death.
If it happened quickly, it would be an cataclysm. If it built slowly, people could adapt in interesting ways.
