Just saw this so it might be too late but perhaps others will have the same question. Here is my take.
1. Ancient tree. (full party) Take out the ancient mephits first. Get a caster / archer on top of one of those pillars and take them out. (If it doesn't feel like cheese to you) the void bulb will pull them into range and or a surface you've created. Burn the twisting vines, this will create fire damage, eliminate the regeneration abilities and allow your melee toons to proceed. Take out any remaining summoned mephits with area spells or cleave.
2. Hobgoblin boss. (no cheese) Kill the other goblins in the temple first, use thunderwave and push to get the bulk of the goblins into the spider pit. Take out the rest using area spells. Use misty step to get away from spider pit once Ragzlin comes in close because he will push you in -- wear him down as much possible with spells and arrows and then take him out.
(cheese) drop a bunch of barrels around the room, get into the rafters and use a fire arrow / cantrip or other fire starter. Interestingly, this is the only way I've found to take out both the tadpole and goblin bosses.
