Hey hey.
I will list a few advices that should be helping you.
1. Play not only with your friends, but also the rest of the community, at least spectate some games, or watch replays. There you will see bether players using very different strategies, that you can copy and use yourself to improve. (You will kill all of your friends ;))
2. When your friend has the possibility to create a huge reaperball, you did something wrong in the first half of the game. The key against tanks is not to let them become so strong. You must face them when they are weak, alone and unsupportet.
3. Use your opponements weakness for yourself. Tanks are awefull slow. (Panthers and Kodachis aren't, but they are weak) So raid, raid, raid. Kill is eco as often as you can. He will not be able to get a huge army with bad eco.
4. When you went for spiders on a semi-flat map where he uses heavy tanks, use the terrain for your advantage = raid with fleas. Flank him with your army, force him to stay in his base with his army....
5. Terraformed Pikes are your friend! 2 Stardusts on a pike make a great defense against 1 or 2 reapers. Same goes for crabes.
6. When you went for shields. Use Dirtbags, they help you to get your friends army stuck, always keep a line of them in front of his army.
7. Get racketeers. Tanks are big slow units, which appear in small groups. (Normaly there should never be more then 5-8 in the endgame. When there are more. You are both waiting way to long to finish the game) So stun them out, then swarm them with bandits or whatever.
8. Use Infiltrators. When he attacks you with a single goli. get 2 or 3 infiltrators and stun him for 30 secends. Kill it then.
9. When you have LV's get ravagers. They aren't as strong as reapers, but cheaper! Ravagers beat reapers! (Also they are faster)
10. Use Domminatrixes. When you have defenses, where he needs a bit of time to break threw. Get a dommiball. They capture his units, and leave them to eliminate themself. (Reatred with those dommis that captured an enemy unit)
11. Farady. Maybe the most underrated defensestructure in the game! They are tanky and they stun your opponemt out so that it cant retreat!
12. Try more different factoryes. There a many more good counters in the game. When you try other playstyles you will surprise your enemy.
Ok i guess this should be helping. One last thing. If you need more advices, i offer you to join us on mumble. We give advices, help eachother ingame, play training rounds etc.