I would like to start off by saying I agree reaching Silver 1 mission needs to be removed or pushed much further down the Missions order, as I write this I met the requirements for the next 42 missions and was only held up by the reach Silver 1. I have now achieved this as a F2P player on Day 35 of the account. Here's how I did it:

My team and their role:

Apothecary - Speed buffer, healer (High Khatun is f2p and better in this role, however I have only had her 5 days so she is not built yet)

Elhain - Nuker. My starter champ (Kael is a better option for this but I did not know this when I started my account).

Warmaiden - Defense Down debuffer, secondary damage dealer (f2p and easy to book by farming her in campaign)

Bellower/Runic Warder - Tank/Utility - This one is optional, anybody can fit this role, I tried a second nuker here with pretty poor results. A 4th guy offering a bit of survivability and support seemed to fair much better. If you don't have Bellower/Runic Warder there are plenty of campaign farmable guys you can stick in here Diabolist/Marked would work.

Gear wise they are all in 3/4/5 star campaign farmed or rewarded gear from quests/tournaments. Most have rings but no banners or amulets. Elhain and Warmaiden are in lifesteal, Apothecary is 3 sets of speed, your tank wants to be in high hp or high def depending which benefits their character most. This is the same gear they wear in campaign/dungeons, I don't have enough gear or silver to switch things in and out.

I also have no champs with full masteries, my great hall has 1 rank in accuracy for all except Force which I upgraded to 3 so Warmaiden can hit her defense down more easily.

This isn't easy as without a speed aura lead I had to be very careful about which fights I picked (if you have a fast high khatun you have way more options available to you). I got to within 1 win of silver 1 on 4 previous occasions on the last 2 weeks only for me to be stomped down to Bronze 3 again, I finally made it today to beat the mission, knowing full well to progress higher I will likely need better champs and gear.

To sum up, it's tough but not impossible to get to Silver 1, I have poor gear, few masteries, no amulets or banners and I lost my temper on 500000 occasions but I finally got there. Avoid speed aura lead fights if they have a defense down champion in their lineup unless you have a speed aura yourself and are confident of being faster.