Another way to possibly obtain an increase in boost for not just troop training but this possibility can be done with trap training, research, and construction as well.
If you take a look at relic gear there are several offering very nice boost however to achieve those nice boost percentages they require you to have legendary level 6 yellow materials. To waste those highest quality materials on relics that only last 30min to an 1hr can be very hard to make yourself do.
Now take a closer at the boost amounts on nearly every single relic gear piece offered at level 2. Nearly all relics offer half the boost at level 2 as they do at level 6.
So for any gear slots that you are not yet getting any boost for either troop or trap training and maybe not as likely but also research and construction. You can find a gear piece for that open slot that offers the boost you are looking for. Once you identify the relic gear offering the boost you seek for the slot you have open you can craft that gear at level 2 and get half the boost that you would at level 6.
If you are already getting a boost from a gear slot then chances are that boost is higher than you will be able to achieve with level 2 relics.
My math may be a little rusty but I believe the following to be correct
THIS MATH is BAD
Provided that you had an ample supply of all the materials for any gear item. You could craft 256 level 2 pieces of hero gear for the same materials as 1 level 6 hero gear.
16 level 1 to combine for a single level 2 64 level 1 to combine for a single level 3 256 level 1 to combine for a single level 4 1024 level 1 to combine for a single level 5 4096 level 1 to combine for a single level 6