You've pretty much covered it afaik. Other than that,

1. You can buy credits for acoins in the store (as a last ditch option if needed). 

2. Another option is to look for any mech achievements milestones that can be met easily that might give crates, fortune keys or direct credits. (Unless this is what you meant by mech missions.)

3. Do you get ads? If yes, on mobile the first few games of each day may reward you with credits if you watch the optional ads.

fortunately no ads :D

Pretty much that's it. There are also a few claimables in the store. Here's the link: https://store.plarium.com/en/mech-arena/

:-) To Plarium's credit, and to my knowledge, watching ads is voluntary unless they've implemented something new lately.

Papa, How do you earn 400k credits in such a short amount of time! Can you help me by telling me what exactly you'd do to accumulate credits? 

I'm grinding for over a month and have currently 640k credits (330k spent in the Overdriver 8 ECR). I watch ads to get 20k credits (I'll try to do it daily for a consistent increment), get the dailys done, play 1-2 tournament matches, complete HS events or other relatively easy events. Is there any other strategy to earn more in-game resources? (forgot to mention I claim a series of rewards from the Plarium Store daily)

well I dont think i do anything special, one thing for sure is the more you play the more you get.. i do everything you do

apart from ads, somehow im lucky i dont have them and that is the main reason why im in this particular mech arena type of game, most other games have adds.. i think there was something called titans.. basically i was installing bunch of games for my son to check what had no ads and kept everything without ads. Because he would then install every game from the ad and play quite ugly games created by brain-rotted people. 

So i have now 2 days left and i still need 270k credits as overdriver just doesnt want to fall into my hands yet :)

Just recently there was event with some sort of very golden massive crate it had 70k credits and blueprints.. Its not common i admit.

I think my credits mainly come from little crates, events and mech achievements, for example i just got Volti and it has these tasks: 

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The first tasks are usually super easy and quick so its like 10k for not much effort. 

I do them for all mechs every day, i have separate hangar just for mech achievements where i would use some M.D. mech with thermal lances to complete 400k damage.. 

I wish i could have an extra hangar because if i get an event saying kill 25 mechs with beam weapons then i need to mess with my hangars and its a pain. BIG PAIN, GIVE ME EXTRA HANGAR PLEASE!

if things go wrong i have 150k backup credits here :)

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Every little crate you getting for a game gives you 500 credits.. i just played one game and got 2 crates so its 1k credits per game.. if you need 100k just play 100 games hahaha..

There is also task with 24k crate as reward, i used to always be able to complete them but when it says "assault weapons from behind" i give up straight away because there must be a bug, i often do like 4 kills from behind and it only counts 1 for some reason, i cant prove it. I hope overdriver will help me get kills done. Its also much more impossible to kill somebody with assault before it turns as everyone is moded now and i need to unload a couple of magazines from reavers for it.. impossible task unnoticed. Plus add bug to it.. in 3 days i make 1 kill at best.. 

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Thanks for your advice :) Still that flanking an opponent with assault weapons is very tough (even if u get overdrivers those won't help you much unless you pick off low-HP bots who seem to be showing their backs to be shot by ur weapons...) 

The game does not force ads on any person (I mean it's completely up to a person for watching ads). I think watching an ad gives me 4k credits per ad which I use thrice, and I watch another ad on the shop (which will be next to blueprint shop on the game) and get 5k there. There's also a mission (that one is classified as a daily task but IDK why won't it count toward "complete 30 daily objectives" weekly task) which will give me 3k once I watch two ads in the game. In total, that's 20k per day. 

There are 5 rewards on the shop which I can claim from the shop by watching ads. But I won't watch every single ad there (I watch the first one only) because we all hate ads (especially if they are intrusive and unskippable) 

you most likely right that overdrivers wont help, its just funny how in the past beam weapons from behind felt impossible and now i realize they are not an issue at all when comparing it to assault weapons from behind.. im sure i completed it once or twice.. but basically i look at reward and think, no its not worth my 3 hours time and even then nothing is for certain

And i mean not 3 hours of normal gameplay, i mean 3 hours of torture idiotic game where i will be running around enemies to do this and a minute swearing for every game that did not count it as a kill from behind