I quite often use a bottom heated gas oven at work for my slow cooked pork joints as we can't afford to have our main ovens on such a low heat all day. Never once have we had an issue getting crispy skin.
Using a blow torch you will probably find the skin blackens on top and the underside still being chewy-er than out of date beef jerky...
If you're meat is burning on the bottom due to the high bottom heat required for crispy skin, make sure you lay your meat on chopped vegetables and a little water or wine. Use the veg for gravy after cooking :-)
If you really really can't get the skin to crisp up. Heat some veg oil in a pan till it's smoking and then carefully pour it over the skin. If you do this after the vegetables and juices have been removed you'll be able to repeat the procedure a few times until your skin is puffed up.
Another way would be to remove the skin after cooking the meat and return it to the oven, skin side down, while the meat rests.
Finally, again once the meat is cooked and resting. Remove the skin and deep fry it for instant "pork scratching" style crackling.