Guys I need your help

for gameplay vlad extra best, but for taste I suggest for Medusa (Lancer)
for archer just both, emiya for AOE & tristan for solo

Have a decent amount of gold lancer and I can tell you that Vlad lancer's kit is very very solid. I have him at 10/10/10 NP5. I have no doubt if I grail him, he would be able to function like a pseudo 5*.

Medusa lily is very underwhelming. The quality between their kits are not even close.

I also don't have SSR lancer XD. But for SR lancer I have Li Shuwen and good ol Eliza Bathory. Also have all 3 star and lower rank lancer. For 3 star and lower OG Cu is good enough to carry you around. Fujimura Jaguar is also quite good for single target lancer. I heard people saying Ana damage is no good until she get her strengthening quest in 2 years. Vlad is better than Ana in this case since he quite tanky and have higher damage than Ana.

You will have 33.33% chances during new year to get one of these 3 SSR lancer during GSSR pickup if you want lancer: - Enkidu, Karna, Artoria Lancer

For Archer, I can say all Archer is good archer. You can try set both in field and use Tristan NPQQ then hopefully you get Emiya brave chain next turn. Use Projection + Hawkeyes. Third turn UBW. This will supply good amount of crit star for your archers to spam crit. For 3 star Archer: -

David: - support Archer with good skill set
Robin: - One turn burst NP damage
Euryale: - Male killer
Billy: - One turn burst crit damage.
Touta: - his skill is more toward tankinest and have AOE buster. Also have 3 turn buster damage up. But since you have Emiya you can ignore him for a while.

For 5 star Archer during GSSR you can get either Orion(Male killer), Tesla, or Arjuna

Medusa Lancer has great HP for a 4-star character as well as one-turn Invincibility and a three-turn Guts combined in one skill, making her quite hardy. Her Charm and NP, provided luck favors you, can stun the enemy to further minimize attacks. In a straight-up fight, however, I find her to be quite underwhelming.

Vlad has an Debuff Resistance, ATK up, DEF up, and heal all rolled into a single skill with only a 5-turn cooldown at max level. He can also buff the whole party's NP damage for a single turn (useful for an NP chain) as well as a Taunt skill. Works very well with a proper team for him to support and be supported by in turn.

Both Medusa and Vlad at ST Lancers so, given the lack of any other option, would be decent enough boss killers, more so for Medusa if the boss is male since she has a chance of charming him.

Emiya makes for a fairly good Crit-type Archer, especially now that his second interlude has come out. And with three Arts cards he can build up with NP pretty quickly in an Arts team with repeated Arts Chains. His NP also generates a lot of quick stars if paired with his Hawkeye and Projection skills. He also comes with a one-turn Evade for a bit of survivability. That said, his overall damage can be quite underwhelming outside of specific situations.

I haven't had a chance to use Tristan just yet, none of my friends have him or put him in their support party, but he does come with an NP gain skill (up to 50% at max level but with a one turn NP Seal preventing you from firing off his NP on the same turn) and a full party Evade skill, similar to David. Tristan's party Evade would be very useful in the Camelot chapter, especially if you can use David as well. Not to mention he would have a class advantage against many of the bosses in that chapter as a lot of them are Sabers.

That said, I think it would be better to level up all of them so you can use them as needed. But if you only have the resources for one or two then it depends on what you need more at the moment.