Hi, diamond adc main here.
- Against malphite ult, you have quite alot of options. His ult has 1000 cast range + 300 from the hitbox so if you stay outside of that range and maintain it, you're fine to fight anyone near you. Get a banshee's veil or guardian angel if you're getting destroyed by his ults. Otherwise, you can use flash to dodge the ult or any dash/gap closer ability (Note: Graves' dash has a faster animation than tristana's rocket jump. You will have to cast your gap closer earlier to account for this delay).
A more direct answer to your question is: It depends. If you're getting destroyed by the malphite ult, then you have no choice but to stay away until you get a defensive item that allows you to fight or unless you have flash or some type of dash to dodge it. If malphite ults you and you don't die in the fight that follows, then you dont have to be scared to engage in these teamfights, as long as you're autoattacking the closest target.
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Group with your teammates for teamfights, especially once laning phase is over. If you see your team grouping , just group with them. This is the best thing you can do if you're inexperienced with these types of situations. Once you get more experience with reading the situation, you can decide to split push or go for another objective depeding on how strong your teammates are. Remember, you're the ADC who is responsible for most of the team's dps late game especially. Do not make a selfish play of going to farm jungle camps or some lane when your teammates are grouped and ready to fight. You can cost your team the game by doing so. (some may argue that its your teammates' fault for fighting without you but if you already knew that, why didn't you just group in the first place? You're in control of what you do in each game.)
If you've just won a teamfight, go for an objective like turrets or dragon. Do NOT chase any stragglers because you're simply wasting your time going for a kill rather than going for an objective that can help you win the game. This is a mistake alot of lower elo players make where they chase the 1 or 2 enemies running around the map instead of going for an objective. The kill benefits only yourself but an objective benefits your entire team and actually helps you win the game.
If you're behind in terms of gold, it really depends on what adc you're playing. If you're playing a heavy autoattack based adc like jinx/caitlyn/tristana, don't group until you have at least infinity edge. If you're playing an AD caster like lucian/corki/ezreal, you want to group up after your first item since ad casters tend to have more dmg in their skill kit.