If you are struggling with leveling up WT's organically whilst playing through the campaign, I'd recommend pausing it for the time being and setting it to WT5 so you can comfortably get through the campaign until level 30. The WT makes no impact on the amount of level exp you get for your character so just sit back and enjoy the campaign after which you will likely be level 28 / 30, have a majority to all of your stat points, and probably enough epics/legendaries dismantled that have Tier 2 / Tier 3 mods to start making a build.
Once I'd made the decision to do this it becomes less of a grind to get to the end and more fun as you aren't constantly fighting over-leveled mobs with under-leveled gear as without farming elites/bosses you'll never have enough resources to keep your gear leveled with the mobs.
After the campaign, my Level 30 Pyromancer is now blasting through the WT's with an LMG & Volcanic Rounds build, and because gear drops more frequently it's easier to upgrade the bits I like the stats/mod rolls on for my playstyle. I am not yet doing expeditions as I'd rather get up to WT15 before tackling the end game so just completing all the side quests, hunts, and assassination missions.