I do not know if you use to this genre, I was pretty comfortable with UI/UX. I wonder if the UX is bad for new player or if it is more related to personal habits.
I started this game on Insane/Ironman, and drop the Ironman mode after loosing on Act 1, Chapter 4. I feel the game spike really hard, and succeed in insane need you to know and use many trick of the game. It is what I except from this kind of difficulty. You have to decide the best move and it is impossible on your first run.
- Gun allows many of your character to do some damage when they stay far and finish some unit low heals without using a main weapon amo. In your screenshot, you should probably switch to your pistol, I guess your hit chance would increase in this case. Since the game throw a lot to you, save 1 reload is really valuable. The ability to stop locust Overwatch is a must use of the pistol.
- About the locust action, unless you have bad positioning, they often move and go Overwatch. It is important to learn to spread enough you team to do not give good overwatch to locus but not too much to avoid isolated gears.
- For the hole, it is not an obligation to close them immediately, you can win without. For this map, the second wave is only 2 of this small guys, so keep little distance is enough. When you keep playing the game you finally learn that grenade are a resource you should use carefully.
For your example, as I wrote before, I think pistol is more efficient at this range and would give you better chance to hit. Also in this case They seems both too far to hit you. And if the closer one can, the grub do not have the damage to one shoot Sid. Also, you could just overwatch close to you, the grub will rush you character and be interrupted by your overwatch.
In your case I would consider first my option to clean the drone, and just keep distance with wretch. If it is not possible, I would retreat in the place of the locust hole, use the heavy to overwatch the wretch side and Sid to overwatch the drone side. Do not know where Diaz is.
The wretch have 330HP, so you one shoot them with 1 full hit for the heavy. One drone need 3 actions to be down, but if you manage an execution after (+1 action) you get back 3 actions (when full team). So with a nice planned turn you can kill 5 of them in one turn.
PS: You can shoot wretch with you weapon on mêlée range without problem, they only interrupt mêlée.