Tactics is only checked on hit, so ideally it would be something you hit on every swing. The closer to 0 weapon skill your target is, the faster you will gain tactics. Also perhaps goes without saying, but the faster the weapon you use, the faster you'll land hits and raise skill. Katana, Dagger, Quarterstaff and Bow I think are the fastest weapons of each type.
I've been hunting monsters for gold with an axe, I tend to use the faster axes (double, axe, executioner's). I suppose using a katana couldn't hurt, just to get 'er done a bit quicker. Thanks
find yourself a Spaniard. I hear they make for great strategists. If Spaniards are in short supply then I recommend as Dalamage suggested and get a faster weapon. If you are gaining swords get a hunting knife or a cleaver and chop up some orcs, ettins, and trolls. (this will result in extra gore splattered upon your tunic seeing how you'll have to make thousands of incisions before they die)
Tactics is slow, but can be GM'd off anything. More hits = more gains. You do not need to hit a difficult target. I managed to GM wrestling (0 to 100) and archery (50 to 100) and ended up with only 91 tactics. Took another 6 hours to get to 100.
If you check my hunting guide, you may notice I've been trying reapers. I have screenshots, just have to write up my quest report I'll just keep hitting stuff, thanks guys