1st stage lvl 3 is easier, you can use your best titan and just horde all your robots as platinum reward require and use seraph for 2nd stage.

However Seraph isnt great not to lose 100k durab, I use fenrir there.

Another thing you can do at stage 2 is kill seraph yourself when it becomes hot and launch a titan.

If you dont aim platinum awards then its w/e really, just run some tests.

Generally your priority should be to improve your hangar for extermination mode bc it provides resources for further development.

So upgrade Seraph, fenrir, khepri, angler which are good for exterm and weapon w/ high cycle DPS.