Marauders: Vanish / CC their defensive cooldowns, Flashbang to heal yourself, kite them using root + cover, cleanse their DoTs. If you have time during a flashbang, drop a freighter flyby on yourself and crouch in it and start winding up Charged Bursts.
Assassins: Don't push them full resolve. Wait until they try to use their uninteruptable lightning, then CC them or vanish. When they use their tech damage resistance cooldown, switch to Quick Shot for the duration. Flashbang to heal.
Essentially, fighting both of these classes is only really difficult if you approach them like you would any other opponent, by trying to dictate the fight. Against these two classes, you have to play more reactively against their cooldowns rather than making them try to counter yours.