Fire and Freeze can be used together effectively, as I've done similar things in the past, however for the sake of switch-shooting I would suggest replacing one of them with something that doesn't have a tendency to cancel the other.
If I'm recon gunning in the same way, I typically use Umbra with Storm. Another popular choice is dual Storm Drivers if you feel like ruining someone's day. I say go for shock since it removes invincibility frames, which means you can land more hits during the spasms if you switch fast enough.
Recon in general is all about the mindgames, and as such I try not to mark people right away if I'm confident I can land a shot before the fight initiates. Once my presence is known I do what I can to give the death mark before proceeding.
Sometime they run. Sometimes they spazz randomly in an attempt to find me. Either way it works, and both outcomes are generally amusing.
You have a few options with pure alchemers, though this is mainly opinion based on personal experience.
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1.) Snipe them from off-screen with charges before going in for the kill. Preferably with shock or freeze, something that will interrupt their movement. If it lands and causes a status, then proceed. If it lands and doesn't, cloak and let them flail around for a bit before trying again. Same for if it misses.
2.) Fire a shot and cloak/move away. This is good for starting a fight out with an advantage to you. It works even better if the first hit causes a status, though it's not always necessary. After landing a hit, move away (or closer, depending) and repeat the process. If you get found out feel free to try and switch-shoot them in their face until one of you drops.
3.) Be a hero and immediately walk up to them while cloaked and try unloading everything you own, switch-shooting without remorse.
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I'm a recon, but not really a recon gunner, so take what you will. Good luck.