As you’ve noticed, trying to build into Mortar Trap and Fire Strike is very point-intensive, even with tons of + skills to Demolitionist 
Anyways, you need/want more Offensive Ability. ~2150, even with Flashbang’s DA strip isn’t great. Try the Survivor’s Ingenuity Augment from Devil’s Crossing. Hawk and Chariot of the Dead are good sources from devotion.
You can afford to cut down on overcapping Blast Shield to 12/12 (or slightly lower like 10/12), Vindictive Flame to 16/16 (or 9/16), Flashbang to 12/12 (or slightly lower like 10/12).
Consider Thermite Mine for resist reduction. Even 1 point for “drop-and-forget”, if something steps on them, great, if not, it didn’t cost you much time.
Devotion-wise, try to fit Elemental Storm in for more resist reduction.
There might be another way you can make a gunner/Mortar Trap build work but keep in mind that it is purely conjecture/untested (lots of explaining stuff ahead):
[spoiler]The idea at a basic level is to stack large amounts of Physical damage converted to Fire damage. The ramifications of focusing on this mean:
- Large amounts of Mortar Trap’s Physical is converted meaning less focus on getting skill levels (besides getting that 2nd Mortar).
- Temper’s flat Physical becomes a secondary source of Fire.
- Your weapons do close to all of their base damage as Fire.
- As a side bonus, Explosive Strike does close to all of it’s Physical damage as Fire.
- Meteor Shower becomes even more potent if you go for it.
The question is how to do this, 2-handers start with 45%, adding in a component adds another 10% and then using an amulet like Empowered Alazra’s Ruby adds another 10% for 65%. Not bad but we want more.
If we use a pair of pistols, they combined have 30% (15% from each weapon), a component in each adds 20% more and finally an amulet adds another 10% for 60%. Worse than a 2-hander.
Well, the answer is, we use a special kind of weapon - Francis’ Gun.
Francis’ Gun is one of the placed rares in Act 1 like Hevill’s Greatsword. Like crafted rares (like Master’s Spellblade), it can roll with one prefix or one suffix but not both. Take a look at this to see how much conversion we can pick up with it.
To sum it up:
- 30% from each Gun, so 60%.
- Another 20% from components.
- 10% from Empowered Alazra’s Ruby.
This nets us an impressive 90% at base, above average rolls give us the potential to go slightly higher or even meet 100% (though this needs really good rolls).
It’s rough but this is what I’ve come with for a final build using your MIs. If you upgrade to AoM, you’ll probably want to go Demolitionist/Inquisitor and look out for the Dagallon set.
[/spoiler]
